A136112 Indices of pentagonal numbers > 0 which are not the difference of two larger pentagonal numbers.

1, 2, 3, 5, 6, 8, 9, 11, 15, 18, 23, 24, 27, 51, 54, 71, 72, 81, 96, 99, 123, 128, 135, 162, 216, 239, 243, 263, 288, 303, 311, 359, 384, 423, 459, 479, 486, 519, 591, 599, 639, 648, 683, 699, 729, 743, 783, 863, 864, 879, 891, 911, 1031, 1103, 1151, 1215, 1431

Offset: 1

Author

M. F. Hasler, Dec 15 2007

Keywords

• nonn

Examples

			a(1..3)=1,2,3 since P(1),P(2),P(3) cannot be written as difference of 2 other pentagonal numbers.
P(4)=22=P(8)-P(7), therefore 4 is not in this sequence.
		

Links

• Donovan Johnson, Table of n, a(n) for n = 1..500

Crossrefs

Cf. A000326, A136113-A136118.

Programs

PARI

P(n)=n*(3*n-1)>>1
isPent(t)=P(sqrtint((t<<1)\3)+1)==t
for( i=1,999,for( j=i+1,(P(i)-1)\3, isPent(P(i)+P(j))&next(2)); print1(i","))

Formula

A number m is in this sequence iff A136114(m) = 0 iff A136115(m) = 0

Extensions

Definition corrected, thanks to a remark from R. J. Mathar, Feb 01 2008

More terms from Donovan Johnson, Apr 22 2008