A376745 - OEIS

A376745

Numbers that are not pentagonal pyramidal numbers.

2, 3, 4, 5, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71

(list; graph; refs; listen; history; text; internal format)

OFFSET

1,1

COMMENTS

Complement of A002411. Numbers not of the form k^2*(k+1)/2.

LINKS

Table of n, a(n) for n=1..67.

Eric Weisstein's World of Mathematics, Pentagonal Pyramidal Number

FORMULA

a(n) = n+m if 2n>m(m-1)(m+2) and a(n) = n+m-1 otherwise where m = floor((2n)^(1/3)).

MATHEMATICA

p=71; l=Floor[(2p)^(1/3)]; Complement[Range[p], Table[n^2 (n + 1)/2, {n, 0, l}]] (* James C. McMahon, Oct 07 2024 *)