A385146 Pentagonal numbers that are five times another pentagonal number.

0, 5, 11310, 55089888425, 121583358792810, 592213938425857857485, 1307015893771450525626150, 6366274445128720143976790614385, 14050364815815600018103892686892130, 68437177312018083317895951729386272287125, 151040819318475127652012254231608319757368350

Offset: 1

Author

Kelvin Voskuijl, Jun 19 2025

Keywords

• nonn

Examples

			P = 11310 is a term since P and P/5 = 2262 are both pentagonal numbers.
		

Crossrefs

Cf. A000326, A076140, A385152.

Programs

PARI

is_a000326(n) = my(s); n==0 || (issquare (24*n+1, &s) && s%6==5);
for(n=0,oo,p=n*(3*n-1)/2; if(p%5==0 && is_a000326(p/5),print1(p,", "))); \\ Joerg Arndt, Jun 19 2025

Formula

a(n) = 5 * A385152(n).