OEIS A000250: Number of Symmetric Reflexive Relations on N Nodes

In this milestone Deep Dive, we tackle OEIS A000250: the count of symmetric reflexive relations on an N‑node set. We spell out what reflexive and symmetric mean in plain terms, why the naïve count 2^(N choose 2) isn’t correct, and how the actual enumeration uses deeper number‑theoretic tools—partitions of N and gcd‑type structure—along with the rich history and references in the OEIS entry. A clean example of how a simple graph‑like question opens up connections between combinatorics and number theory, with notes on history, definitions, and related sequences.

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