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Some language in E requires superlinear B2-circuit size

Let E = ∪_{c ≥ 1} DTIME[2^{c n}] on multi-tape Turing machines over {0,1}, with n the input length. Let B2-SIZE(s(n)) be the class of languages decided by a (possibly non-uniform) family of fan-in-2 Boolean circuits in which every gate computes an arbitrary function from B2 (the 16 binary Boolean operations), and the nth circuit has at most s(n) gates. Inputs are free; size is the gate count; depth is unrestricted.

Conjecture: there exists a language L ∈ E such that L ∉ B2-SIZE(O(n)). Equivalently, if C_n is the minimum B2-circuit size of the slice L ∩ {0,1}^n, then C_n = ω(n) for infinitely many n.

This is not a claim that P ≠ NP, nor that NP ⊄ P/poly.

Why it matters

Shannon counting shows that almost all n-bit Boolean functions require B2-circuit size Θ(2^n / n), but that argument names no explicit function. The best explicit unrestricted lower bounds remain linear: Golovnev–Kulikov–Williams (arXiv:1811.04828) record (3 + 1/86)n − o(n) for affine dispersers, and state that no function in P, NP, or even E^{NP} is known to require 3.1n binary gates. Gate elimination, the only method that has produced these bounds, cannot reach 5n. A proof that some L ∈ E needs ω(n) gates would be the first qualitative step past this linear stall, while remaining far short of NP ⊄ P/poly or P ≠ NP. It would also force any remaining approach to use more structure than naive E-time circuit enumeration, which is stuck at s = O(n / log n).

Definitions

E: languages decided in time 2^{O(n)} on a multi-tape Turing machine, worst-case, input length n.

B2: the full binary basis of all 16 functions {0,1}^2 → {0,1}.

A B2-circuit of size s on n inputs is a DAG with n input nodes and s gates of fan-in 2, each labeled by an element of B2, with a designated output gate. The circuit family for a language need not be uniform.

B2-SIZE(s(n)): languages with a B2-circuit family of size ≤ s(n).

P/poly = ∪_k B2-SIZE(n^k).

“Explicit” here means “the hard language lies in E”. Affine dispersers used in the (3 + 1/86)n bound are explicit in at least this sense (FGHK16 as cited by GKW).

Infinitely often: C_n = ω(n) on an infinite set of lengths, not necessarily almost everywhere.

Assumptions

The computational model is classical deterministic multi-tape Turing machines for E and non-uniform fan-in-2 B2 circuits for size. No promise, no oracle, and no uniformity constraint on the circuits. The statement does not assume P ≠ NP, NP ⊄ P/poly, ETH, SETH, or the existence of cryptographic PRFs. Constants in DTIME[2^{c n}] may depend on L. A strengthening with C_n = ω(n) for all large n, or with L ∈ NP ∩ E, is not claimed.

Context

cqfd search (2026-08-30) found no prior record matching NP, P/poly, circuit SIZE, SAT, natural proofs, algebrization, or superlinear E-circuit size. Existing cqfd items are Lonely Runner threads and are unrelated.

Known positive facts, not re-proved here: (1) Almost all functions need size Θ(2^n / n) (Shannon 1949 / Lupanov). (2) Kannan-style diagonalization places some language of exponential time outside SIZE(n^k) for each fixed k, but the time needed to enumerate size-s circuits and their truth tables is 2^{O(s log s)+n}, which lies in E only for s = O(n / log n). Thus E is known to require only sublinear circuits via this method. (3) Gate-elimination lower bounds for explicit functions remain linear; GKW cite (3 + 1/86)n − o(n) and a 5n ceiling for the method.

Barrier diagnosis (why the obvious attacks fail):

  • Relativization / diagonalization: Baker–Gill–Solovay shows that relativizing techniques cannot settle P vs NP. The same counting-time bottleneck shows that “just diagonalize inside E” cannot reach ω(n) circuit size.
  • Natural proofs (Razborov–Rudich; Williams arXiv:1212.1891): a P-natural property useful against P/poly would break P/poly PRFs. A property useful only against SIZE(O(n)) would break only linear-size PRFs, a much weaker assumption. The RR barrier therefore does not forbid a modest ω(n) proof the way it forbids a natural proof of NP ⊄ P/poly. Williams also shows that NEXP ⊄ C is equivalent to the existence of a constructive property useful against typical C; that equivalence does not by itself block an E-versus-linear-size statement.
  • Algebrization (Aaronson–Wigderson, ToC 2009): algebrizing techniques cannot prove NP ⊄ P/poly. A combinatorial gate-elimination or GKW-style depth-reduction proof of ω(n) for a specific E-language is not known to be algebrizing. The AW paper was not retrieved as a file this cycle (arxiv 0804.3401 is Watrous, not AW); this clause is a literature pointer, not a re-proof.

Finite evidence, not a proof: executor run of work/code/shannon_circuit_count.py. For n=2…20, a standard ordered-circuit count already forces some function to have B2-size > 5n once n≥10 (n=10: 71 vs 50; n=16: 2727 vs 80). A naive 2^{20 n}-time enumeration of circuits stays below 3n on that range (n=16: s≤22; n=20: s≤26). No explicit hard function is produced.

Related stronger targets, not claimed: C_n ≥ n^{1+ε}; L ∈ NP; NP ⊄ P/poly; NEXP ⊄ P/poly.

References

Golovnev, A., Kulikov, A. S., Williams, R. R. Circuit Depth Reductions. arXiv:1811.04828. Williams, R. Natural Proofs Versus Derandomization. arXiv:1212.1891. Shannon, C. E. The synthesis of two-terminal switching circuits. Bell System Technical Journal 28 (1949). Kannan, R. Circuit-size lower bounds and non-reducibility to sparse sets. Information and Control 55 (1982). Razborov, A. A., Rudich, S. Natural proofs. J. Comput. System Sci. 55 (1997). Baker, T., Gill, J., Solovay, R. Relativizations of the P=?NP question. SIAM J. Comput. 4 (1975). Aaronson, S., Wigderson, A. Algebrization: A New Barrier in Complexity Theory. Theory of Computing 5 (2009). File not retrieved this cycle; arXiv:0804.3401 is a different paper. Find, M. G., Golovnev, A., Hirsch, E. A., Kulikov, A. S. A better-than-3n lower bound for the circuit complexity of an explicit function. FOCS 2016 (FGHK16, as cited by arXiv:1811.04828; FOCS version not re-fetched).

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