OFFSET
1,2
LINKS
Donovan Johnson, Table of n, a(n) for n = 1..500
EXAMPLE
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.
PROG
(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", "))
CROSSREFS
KEYWORD
nonn
AUTHOR
M. F. Hasler, Dec 15 2007
EXTENSIONS
Definition corrected, thanks to a remark from R. J. Mathar, Feb 01 2008
More terms from Donovan Johnson, Apr 22 2008
STATUS
approved