OEIS A000326: Pentagonal numbers

A deep dive into the pentagonal numbers A000326, tracing their geometric roots and arithmetic formula P(n) = n(3n−1)/2, including the pentagonal test x is pentagonal iff (sqrt(24x+1)+1)/6 is an integer. We explore generalized pentagonal numbers from negative n, Euler’s pentagonal number theorem and their role in partitions, and the striking links to primes: for any prime p>3, p^2−1 is divisible by 24 and corresponds to generalized pentagonal values. We’ll connect the geometry of dots forming pentagons, modular patterns like primes of the form 6n±1, and the unity of patterns that appear across geometry, arithmetic, and combinatorics, showing why pentagonal numbers keep surprising us.

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