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Combinatorics

Deterministic nonbipartite Ramanujan graphs in every fixed degree

OpenAI research collection · 1 manuscript · Source checked 2026-10-19

OpenAI's result summary

For every fixed d ≥ 3, constructs a simple d-regular nonbipartite Ramanujan graph on every sufficiently large even number n of vertices, with every nonconstant adjacency eigenvalue strictly between -2\sqrt{d-1} and 2\sqrt{d-1}. A deterministic algorithm outputs the full adjacency list in polynomial bit time, with exponent depending on d.

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Short excerpts below come from OpenAI's abstracts. The full abstract and original PDF preserve the paper's precise scope.

MANUSCRIPT 01

Deterministic nonbipartite Ramanujan graphs in every fixed degree

For every fixed integer d ≥ 3, we give a deterministic algorithm that constructs a simple nonbipartite d-regular Ramanujan graph on every sufficiently large even number n of vertices. It outputs the full adjacency list in polynomially many bit operations, with an exponent that may depend on d. Every nonconstant adjacency…

Full source abstract

For every fixed integer d ≥ 3, we give a deterministic algorithm that constructs a simple nonbipartite d-regular Ramanujan graph on every sufficiently large even number n of vertices. It outputs the full adjacency list in polynomially many bit operations, with an exponent that may depend on d. Every nonconstant adjacency eigenvalue lies strictly between -2\sqrt{d-1} and 2\sqrt{d-1}.

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