20826

Appears in sequences

• 4-dimensional analog of centered polygonal numbers. Also number of regions created by sides and diagonals of a convex n-gon in general position.at n=28A006522
• 9-gonal (or enneagonal) pyramidal numbers: a(n) = n*(n+1)*(7*n-4)/6.at n=26A007584
• Numbers k such that sigma(k) = Sum_{j|k, j<k} sigma(j).at n=3A066218
• Smallest integer > 1 which is both n-gonal and centered n-gonal.at n=32A072277
• Second right hand column of the Beta triangle A160480.at n=23A160483
• a(n) = n*(n^2 - 4*n + 5)/2.at n=36A162607
• Second elementary symmetric function of the first n terms of (1,2,2,3,3,4,4,5,5...).at n=25A203298
• Even 9-gonal (nonagonal) pyramidal numbers.at n=18A218329
• Numbers k such that sigma(k) divides Sum_{d|k} sigma(d).at n=5A221219
• Shiraishi numbers: a parametrized family of solutions c to the Diophantine equation a^3 + b^3 + c^3 = d^3 with d = c+1.at n=25A226903
• Numbers k such that if x = Sum_{j|k, j<k} (sigma(j) - j) then k = Sum_{j|x, j<x} (sigma(j) - j).at n=5A238765
• 35-gonal numbers: a(n) = n*(33*n-31)/2.at n=36A282851
• a(n) = s(n,n) + s(n,n-1) + s(n,n-2), where s(n,k) are the unsigned Stirling numbers of the first kind (see A132393).at n=20A308305
• Let PG(n) be the graph with one node for each free n-celled polyomino in the {4,5} tessellation of the hyperbolic plane, and edges between nodes corresponding to polyominoes that can be obtained from each other by moving one cell, where the intermediate polyform (the set of cells remaining when the cell to be moved is detached) is required to be a connected polyomino. a(n) is the number of edges in PG(n).at n=7A390203