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Abstract: Abstract We introduce new forms of attack on expander-based cryptography, and in particular on Goldreich’s pseudorandom generator and one-way function. Our attacks exploit low circuit complexity of the underlying expander’s neighbor function and/or of the local predicate. Our two key conceptual contributions are: We put forward the possibility that the choice of expander matters in expander-based cryptography. In particular, using expanders whose neighbor function has low circuit complexity might compromise the security of Goldreich’s PRG and OWF in certain settings. We show that the security of Goldreich’s PRG and OWF over arbitrary expanders is closely related to two other long-standing problems: The existence of unbalanced lossless expanders with low-complexity neighbor function, and limitations on circuit lower bounds (i.e., natural proofs). In particular, our results further motivate the investigation of affine/local unbalanced lossless expanders and of average-case lower bounds against DNF-XOR circuits. We prove two types of technical results. First, in the regime of quasipolynomial stretch (in which the output length of the PRG and the running time of the distinguisher are quasipolynomial in the seed length) we unconditionally break Goldreich’s PRG, when instantiated with a specific expander whose existence we prove, and for a class of predicates that match the parameters of the currently-best “hard” candidates. Secondly, conditioned on the existence of expanders whose neighbor functions have extremely low circuit complexity, we present attacks on Goldreich’s PRG in the regime of polynomial stretch. As one corollary, conditioned on the existence of the foregoing expanders, we show that either the parameters of natural properties for several constant-depth circuit classes cannot be improved, even mildly; or Goldreich’s PRG is insecure in the regime of a large polynomial stretch for some expander graphs, regardless of the predicate used. PubDate: 2022-03-16

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Abstract: Abstract We provide a complete picture of the extent to which amplification of success probability is possible for randomized algorithms having access to one NP oracle query, in the settings of two-sided, onesided, and zero-sided error. We generalize this picture to amplifying one-query algorithms with q-query algorithms, and we show our inclusions are tight for relativizing techniques. PubDate: 2022-02-05 DOI: 10.1007/s00037-021-00219-w

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Abstract: Abstract We study the complexity of approximating the partition function of the q-state Potts model and the closely related Tutte polynomial for complex values of the underlying parameters. Apart from the classical connections with quantum computing and phase transitions in statistical physics, recent work in approximate counting has shown that the behaviour in the complex plane, and more precisely the location of zeros, is strongly connected with the complexity of the approximation problem, even for positive real-valued parameters. Previous work in the complex plane by Goldberg and Guo focused on q = 2, which corresponds to the case of the Ising model; for q > 2, the behaviour in the complex plane is not as well understood and most work applies only to the real-valued Tutte plane. Our main result is a complete classification of the complexity of the approximation problems for all non-real values of the parameters, by establishing #P-hardness results that apply even when restricted to planar graphs. Our techniques apply to all q \(\geq\) 2 and further complement/refine previous results both for the Ising model and the Tutte plane, answering in particular a question raised by Bordewich, Freedman, Lovász and Welsh in the context of quantum computations. PubDate: 2022-02-03 DOI: 10.1007/s00037-021-00218-x

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Abstract: Abstract We prove that the border rank of the Kronecker square of the little Coppersmith–Winograd tensor \(T_{cw,q}\) is the square of its border rank for \(q > 2\) and that the border rank of its Kronecker cube is the cube of its border rank for \(q > 4\) . This answers questions raised implicitly by Coppersmith & Winograd (1990, §11) and explicitly by Bläser (2013, Problem 9.8) and rules out the possibility of proving new upper bounds on the exponent of matrix multiplication using the square or cube of a little Coppersmith–Winograd tensor in this range. In the positive direction, we enlarge the list of explicit tensors potentially useful for Strassen's laser method, introducing a skew-symmetric version of the Coppersmith–Winograd tensor, \(T_{skewcw,q}\) . For \(q = 2\) , the Kronecker square of this tensor coincides with the \(3\times 3\) determinant polynomial, \(\det_{3} \in \mathbb{C}^{9} \otimes \mathbb{C}^{9} \otimes \mathbb{C}^{9}\) , regarded as a tensor. We show that this tensor could potentially be used to show that the exponent of matrix multiplication is two. We determine new upper bounds for the (Waring) rank and the (Waring) border rank of \(\det_3\) , exhibiting a strict submultiplicative behaviour for \(T_{skewcw,2}\) which is promising for the laser method. We establish general results regarding border ranks of Kronecker powers of tensors, and make a detailed study of Kronecker squares of tensors in \(\mathbb{C}^{3} \otimes \mathbb{C}^{3} \otimes \mathbb{C}^{3}\) . PubDate: 2021-12-18 DOI: 10.1007/s00037-021-00217-y

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Abstract: Abstract Given a multivariate polynomial computed by an arithmetic branching program (ABP) of size s, we show that all its factors can be computed by arithmetic branching programs of size poly(s). Kaltofen gave a similar result for polynomials computed by arithmetic circuits. The previously known best upper bound for ABP-factors was poly \( (s^{ {\rm \log} s}) \) . PubDate: 2021-10-15 DOI: 10.1007/s00037-021-00215-0

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Abstract: Abstract We study the complexity of representing polynomials by arithmetic circuits in both the commutative and the non-commutative settings. Our approach goes through a precise understanding of the more restricted setting where multiplication is not associative, meaning that we distinguish (xy)z from x(yz). Our first and main conceptual result is a characterization result: We show that the size of the smallest circuit computing a given non-associative polynomial is exactly the rank of a matrix constructed from the polynomial and called the Hankel matrix. This result applies to the class of all circuits in both commutative and non-commutative settings, and can be seen as an extension of the seminal result of Nisan giving a similar characterization for non-commutative algebraic branching programs. The study of the Hankel matrix provides a unifying approach for proving lower bounds for polynomials in the (classical) associative setting. Our key technical contribution is to provide generic lower bound theorems based on analyzing and decomposing the Hankel matrix. We obtain significant improvements on lower bounds for circuits with many parse trees, in both (associative) commutative and non-commutative settings, as well as alternative proofs of recent results proving superpolynomial and exponential lower bounds for different classes of circuits as corollaries of our characterization and decomposition results. PubDate: 2021-10-08 DOI: 10.1007/s00037-021-00214-1

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Abstract: Abstract This paper is motivated by seeking the exact complexity of resolution refutation of Tseitin formulas. We prove that the size of any regular resolution refutation of a Tseitin formula \( {\rm T}(G, c)\) based on a connected graph \({G} =(V, E)\) is at least \(2^{\Omega({\rm tw}(G)/ \log V )}\) , where \({\rm tw}(G)\) denotes the treewidth of a graph G. For constant-degree graphs, there is a known upper bound \(2^{\mathcal{O}({\rm tw}(G))}{\rm poly}( V )\) (Alekhnovich & Razborov Comput. Compl. 2011; Galesi, Talebanfard & Torán ACM Trans. Comput. Theory 2020), so our lower bound is tight up to a logarithmic factor in the exponent. Our proof consists of two steps. First, we show that any regular resolution refutation of an unsatisfiable Tseitin formula \({\rm T}(G, c) \) of size S can be converted to a read-once branching program computing a satisfiable Tseitin formula \({\rm T}(G,c')\) of size \(S^{{\mathcal{O}}({\rm log} V )}\) and this bound is tight. Second, we give the exact characterization of the nondeterministic read-once branching program (1-NBP) complexity of satisfiable Tseitin formulas in terms of structural properties of underlying graphs. Namely, we introduce a new graph measure, the component width (compw) and show that the size of a minimal \({1\text{-}\mathrm{NBP}}\) computing a satisfiable Tseitin formula \({\rm T}(G,c')\) based on a graph \({G} = (V, E)\) equals \(2^{compw}(G)\) up to a polynomial factor. Then we show that \(\Omega({\rm tw}(G)) \le {\rm compw}(G) \le {\mathcal{O}}({\rm tw}(G){\rm log}( V ))\) and both of these bounds are tight. The lower bound improves the recent result by Glinskih & Itsykson (Theory Comput. Syst. 2021). PubDate: 2021-08-27 DOI: 10.1007/s00037-021-00213-2

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Abstract: Abstract Interactive proofs of proximity allow a sublinear-time verifier to check that a given input is close to the language, using a small amount of communication with a powerful (but untrusted) prover. In this work, we consider two natural minimally interactive variants of such proofs systems, in which the prover only sends a single message, referred to as the proof. The first variant, known as MA-proofs of Proximity (MAP), is fully non-interactive, meaning that the proof is a function of the input only. The second variant, known as AM-proofs of Proximity (AMP), allows the proof to additionally depend on the verifier's (entire) random string. The complexity of both MAPs and AMPs is the total number of bits that the verifier observes—namely, the sum of the proof length and query complexity. Our main result is an exponential separation between the power of MAPs and AMPs. Specifically, we exhibit an explicit and natural property \(\Pi\) that admits an AMP with complexity \(O(\log n)\) , whereas any MAP for \(\Pi\) has complexity \(\tilde{\Omega}(n^{1/4})\) , where n denotes the length of the input in bits. Our MAP lower bound also yields an alternate proof, which is more general and arguably much simpler, for a recent result of Fischer et al. (ITCS, 2014). Also, Aaronson (Quantum Information & Computation 2012) has shown a \(\Omega(n^{1/6})\) QMA lower bound for the same property \(\Pi\) . Lastly, we also consider the notion of oblivious proofs of proximity, in which the verifier's queries are oblivious to the proof. In this setting, we show that AMPs can only be quadratically stronger than MAPs. As an application of this result, we show an exponential separation between the power of public and private coin for oblivious interactive proofs of proximity. PubDate: 2021-08-18 DOI: 10.1007/s00037-021-00212-3

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Abstract: Abstract We study the approximation of halfspaces \(h:\{0,1\}^n\to\{0,1\}\) in the infinity norm by polynomials and rational functions of any given degree. Our main result is an explicit construction of the “hardest” halfspace, for which we prove polynomial and rational approximation lower bounds that match the trivial upper bounds achievable for all halfspaces. This completes a lengthy line of work started by Myhill and Kautz (1961). As an application, we construct a communication problem that achieves essentially the largest possible separation, of O(n) versus \(2^{-\Omega(n)}\) , between the sign-rank and discrepancy. Equivalently, our problem exhibits a gap of log n versus \(\Omega(n)\) between the communication complexity with unbounded versus weakly unbounded error, improving quadratically on previous constructions and completing a line of work started by Babai, Frankl, and Simon (FOCS 1986). Our results further generalize to the k-party number-on-the-forehead model, where we obtain an explicit separation of log n versus \(\Omega(n/4^{n})\) for communication with unbounded versus weakly unbounded error. PubDate: 2021-08-03 DOI: 10.1007/s00037-021-00211-4

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Abstract: Abstract We investigate the power of randomness in two-party communication complexity. In particular, we study the model where the parties can make a constant number of queries to a function that has an efficient one-sided-error randomized protocol. The complexity classes defined by this model comprise the Randomized Boolean Hierarchy, which is analogous to the Boolean Hierarchy but defined with one-sidederror randomness instead of nondeterminism. Our techniques connect the Nondeterministic and Randomized Boolean Hierarchies, and we provide a complete picture of the relationships among complexity classes within and across these two hierarchies. In particular, we prove that the Randomized Boolean Hierarchy does not collapse, and we prove a query-to-communication lifting theorem for all levels of the Nondeterministic Boolean Hierarchy and use it to resolve an open problem stated in the paper by Halstenberg and Reischuk (CCC 1988) which initiated the study of this hierarchy. PubDate: 2021-07-02 DOI: 10.1007/s00037-021-00210-5

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Abstract: Authors would like to correct the error in their publication. PubDate: 2021-06-10 DOI: 10.1007/s00037-021-00208-z

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Abstract: Abstract We look at the problem of blackbox polynomial identity testing (PIT) for the model of read-once oblivious algebraic branching programs (ROABP), where the number of variables is logarithmic to the input size of ROABP. We restrict width of ROABP to a constant and study the more general sum-of-ROABPs model. This model is nontrivial due to the arbitrary individual-degree. We give the first poly( \(s\) )-time blackbox PIT for sum of constant-many, size- \(s\) , \(O(log s)\) -variate constant-width ROABPs. The previous best for this model was quasi-polynomial time (Gurjar et al, CCC'15; Computational Complexity'17) which is comparable to brute-force in the log-variate setting. We also show that we can work with unbounded-many such ROABPs if each ROABP computes a homogeneous polynomial (or more generally for degree-preserving sums). We also give poly-time PIT for the border. We introduce two new techniques, both of which also work for the border version of the stated models. (1) The leading-degree-part of an ROABP can be made syntactically homogeneous in the same width. (2) There is a direct reduction from PIT of sum-of-ROABPs to PIT of single ROABP (over any field). Our methods improve the time complexity for PIT of sum-of-ROABPs in the log-variate regime. PubDate: 2021-06-10 DOI: 10.1007/s00037-021-00209-y

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Abstract: Abstract We show a new connection between the clause space measure in tree-like resolution and the reversible pebble game on graphs. Using this connection, we provide several formula classes for which there is a logarithmic factor separation between the clause space complexity measure in tree-like and general resolution. We also provide upper bounds for tree-like resolution clause space in terms of general resolution clause and variable space. In particular, we show that for any formula F, its tree-like resolution clause space is upper bounded by space \((\pi)\) \((\log({\rm time}(\pi))\) , where \(\pi\) is any general resolution refutation of F. This holds considering as space \((\pi)\) the clause space of the refutation as well as considering its variable space. For the concrete case of Tseitin formulas, we are able to improve this bound to the optimal bound space \((\pi)\log n\) , where n is the number of vertices of the corresponding graph PubDate: 2021-05-01 DOI: 10.1007/s00037-021-00206-1

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Abstract: Abstract We study the problem of constructing explicit families of matrices which cannot be expressed as a product of a few sparse matrices. In addition to being a natural mathematical question on its own, this problem appears in various incarnations in computer science; the most significant being in the context of lower bounds for algebraic circuits which compute linear transformations, matrix rigidity and data structure lower bounds. We first show, for every constant d, a deterministic construction in time \({\rm exp}(n^{1-\Omega(1/d)})\) of a family \(\{M_n\}\) of \(n \times n\) matrices which cannot be expressed as a product \(M_n = A_1 \cdots A_d\) where the total sparsity of \(A_1,\ldots,A_d\) is less than \(n^{1+1/(2d)}\) . In other words, any depth-d linear circuit computing the linear transformation \(M_n\cdot {\bf x}\) has size at least \(n^{1+\Omega(1/d)}\) . The prior best lower bounds for this problem were barely super-linear, and were obtained by a long line of research based on the study of super-concentrators. We improve these lower bounds at the cost of a blow up in the time required to construct these matrices. Previously, however, such constructions were not known even in time \(2^{O(n)}\) with the aid of an NP oracle. We then outline an approach for proving improved lower bounds through a certain derandomization problem, and use this approach to prove asymptotically optimal quadratic lower bounds for natural special cases, which generalize many of the common matrix decompositions. PubDate: 2021-04-02 DOI: 10.1007/s00037-021-00205-2

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Abstract: Abstract For any finite Galois field extension K/F, with Galois group G = Gal (K/F), there exists an element \(\alpha \in \) K whose orbit \(G\cdot\alpha\) forms an F-basis of K. Such an \(\alpha\) is called a normal element, and \(G\cdot\alpha\) is a normal basis. We introduce a probabilistic algorithm for testing whether a given \(\alpha \in\) K is normal, when G is either a finite abelian or a metacyclic group. The algorithm is based on the fact that deciding whether \(\alpha\) is normal can be reduced to deciding whether \(\sum_{g \in G} g(\alpha)g \in\) K[G] is invertible; it requires a slightly subquadratic number of operations. Once we know that \(\alpha\) is normal, we show how to perform conversions between the power basis of K/F and the normal basis with the same asymptotic cost. PubDate: 2021-03-02 DOI: 10.1007/s00037-020-00204-9

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Abstract: Abstract We establish an exactly tight relation between reversible pebblings of graphs and Nullstellensatz refutations of pebbling formulas, showing that a graph G can be reversibly pebbled in time t and space s if and only if there is a Nullstellensatz refutation of the pebbling formula over G in size t + 1 and degree s (independently of the field in which the Nullstellensatz refutation is made). We use this correspondence to prove a number of strong size-degree trade-offs for Nullstellensatz, which to the best of our knowledge are the first such results for this proof system. PubDate: 2021-02-12 DOI: 10.1007/s00037-020-00201-y

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Abstract: Abstract A stochastic code is a pair of encoding and decoding procedures (Enc, Dec) where \({{\rm Enc} : \{0, 1\}^{k} \times \{0, 1\}^{d} \rightarrow \{0, 1\}^{n}}\) . The code is (p, L)-list decodable against a class \(\mathcal{C}\) of “channel functions” \(C : \{0,1\}^{n} \rightarrow \{0,1\}^{n}\) if for every message \(m \in \{0,1\}^{k}\) and every channel \(C \in \mathcal{C}\) that induces at most pn errors, applying Dec on the “received word” C(Enc(m,S)) produces a list of at most L messages that contain m with high probability over the choice of uniform \(S \leftarrow \{0, 1\}^{d}\) . Note that both the channel C and the decoding algorithm Dec do not receive the random variable S, when attempting to decode. The rate of a code is \(R = k/n\) , and a code is explicit if Enc, Dec run in time poly(n). Guruswami and Smith (Journal of the ACM, 2016) showed that for every constants \(0 < p < \frac{1}{2}, \epsilon > 0\) and \(c > 1\) there exist a constant L and a Monte Carlo explicit constructions of stochastic codes with rate \(R \geq 1-H(p) - \epsilon\) that are (p, L)-list decodable for size \(n^c\) channels. Here, Monte Carlo means that the encoding and decoding need to share a public uniformly chosen \({\rm poly}(n^c)\) bit string Y, and the constructed stochastic code is (p, L)-list decodable with high probability over the choice of Y. Guruswami and Smith pose an open problem to give fully explicit (that is not Monte Carlo) explicit codes with the same parameters, under hardness assumptions. In this paper, we resolve this open problem, using a minimal assumption: the existence of poly-time computable pseudorandom generators for small circuits, which follows from standard complexity assumptions by Impagliazzo and Wigderson (STOC 97). Guruswami and Smith also asked to give a fully explicit unconditional constructions with the same parameters against \(O(\log n)\) -space online channels. (These are channels that have space \(O(\log n)\) and are allowed to read the input codeword in one pass.) We also resolve this open problem. Finally, we consider a tighter notion of explicitness, in which the running time of encoding and list-decoding algorithms does not increase, when increasing the complexity of the channel. We give explicit constructions (with rate approaching \(1 - H(p)\) for every \(p \leq p_{0}\) for some \(p_{0} >0\) ) for channels that are circuits of size \(2^{n^{\Omega(1/d)}}\) and depth d. Here, the running time of encoding and decoding is a polynomial that does not depend on the dept... PubDate: 2021-01-20 DOI: 10.1007/s00037-020-00203-w

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Abstract: Abstract Resolution over linear equations is a natural extension of the popular resolution refutation system, augmented with the ability to carry out basic counting. Denoted \({\rm Res}({\rm lin}_R)\) , this refutation system operates with disjunctions of linear equations with Boolean variables over a ring R, to refute unsatisfiable sets of such disjunctions. Beginning in the work of Raz & Tzameret (2008), through the work of Itsykson & Sokolov (2020) which focused on tree-like lower bounds, this refutation system was shown to be fairly strong. Subsequent work (cf. Garlik & Kołodziejczyk 2018; Itsykson & Sokolov 2020; Krajícek 2017; Krajícek & Oliveira 2018) made it evident that establishing lower bounds against general \({\rm Res}({\rm lin}_R)\) refutations is a challenging and interesting task since the system captures a ``minimal'' extension of resolution with counting gates for which no super-polynomial lower bounds are known to date. We provide the first super-polynomial size lower bounds against general (dag-like) resolution over linear equations refutations in the large characteristic regime. In particular, we prove that the subset-sum principle \(1+\sum\nolimits_{i=1}^{n}2^i x_i = 0\) requires refutations of exponential size over \(\mathbb{Q}\) . We use a novel lower bound technique: We show that under certain conditions every refutation of a subset-sum instance \(f=0\) must pass through a fat clause consisting of the equation \(f=\alpha\) for every \(\alpha\) in the image of f under Boolean assignments, or can be efficiently reduced to a proof containing such a clause. We then modify this approach to prove exponential lower bounds against tree-like refutations of any subset-sum instance that depends on n variables, hence also separating tree-like from dag-like refutations over the rationals. We then turn to the finite fields regime, showing that the work of Itsykson & Sokolov (2020), where tree-like lower bounds over \(\mathbb{F}_2\) were obtained, can be carried over and extended to every finite field. We establish new lower bounds and separations as follows: (i) For every pair of distinct primes \(p,q\) , there exist CNF formulas with short tree-like refutations in \({\rm Res}({\rm lin}{\mathbb{F}_p})\) that require exponential-size tree-like \({\rm Res}({\rm lin}{\mathbb{F}_q})\) refutations; (ii) random k-CNF formulas require exponential-size tree-like \({\rm Res}({\rm lin}{\mathbb{F}_p})\) refutations, for every prime p and constant k; and (iii) exponential-size lower bounds for tree-like \({\rm Res}({\rm lin}{\mathbb{F}})\) refutations of the pigeonhole principle, for every field \(\mathbb{F}\) . PubDate: 2021-01-08 DOI: 10.1007/s00037-020-00202-x

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Abstract: Abstract Probabilistically checkable proofs (PCPs) can be verified based only on a constant amount of random queries, such that any correct claim has a proof that is always accepted, and incorrect claims are rejected with high probability (regardless of the given alleged proof). We consider two possible features of PCPs: \(\circ \quad\) A PCP is strong if it rejects an alleged proof of a correct claimwith probability proportional to its distance from some correctproof of that claim. \(\circ \quad\) A PCP is smooth if each location in a proof is queried with equalprobability. We prove that all sets in \(\mathcal{NP}\) have PCPs that are both smooth andstrong, are of polynomial length and can be verified based on a constantnumber of queries. This is achieved by following the proof of thePCP theorem of Arora et al. (JACM 45(3):501–555, 1998), providing astronger analysis of the Hadamard and Reed–Muller based PCPs anda refined PCP composition theorem. In fact, we show that any set in \(\mathcal{NP}\) has a smooth strong canonical PCP of Proximity (PCPP), meaningthat there is an efficiently computable bijection of \(\mathcal{NP}\) witnesses to correct proofs. This improves on the recent construction of Dinur et al. (in: Blum (ed) 10th innovations in theoretical computer science conference, ITCS, San Diego, 2019) of PCPPs that are strong canonical but inherently non-smooth. Our result implies the hardness of approximating the satisfiability of “stable” 3CNF formulae with bounded variable occurrence, where stable means that the number of clauses violated by an assignment is proportional to its distance from a satisfying assignment (in the relative Hamming metric). This proves a hypothesis used in the work of Friggstad, Khodamoradi and Salavatipour (in: Chan (ed) Proceedings of the 30th annual ACM-SIAM symposium on discrete algorithms, SODA, San Diego, 2019), suggesting a connection between the hardness of these instances and other stable optimization problems. PubDate: 2021-01-06 DOI: 10.1007/s00037-020-00199-3