4,1

Table of n, a(n) for n=4..80.

Eric Weisstein's World of Mathematics, Pyramidal Number

Index to sequences related to pyramidal numbers

a(18) = 5 since 18 is a pentagonal pyramidal number, but not a square pyramidal or tetrahedral number.

Cf. A080851, A176774, A261720.

Sequence in context: A088243 A154660 A004492 * A140255 A050270 A202307

Adjacent sequences: A333911 A333912 A333913 * A333915 A333916 A333917

nonn

Ilya Gutkovskiy, Apr 09 2020

approved