OFFSET
1,1
COMMENTS
A135768 resp. A135769 are subsequences of A136112 resp. A136113; the present sequence gives the indices of the elements of the former which are not in the latter: A136113(A135771(k)), k=1,2,3,... are the pentagonal numbers P(m) which are not the difference of two pentagonal numbers P(n)-P(q) with n,q>m, but only with n>m>q. A136112(A135771(k)) are the corresponding indices of the pentagonal numbers.
LINKS
Donovan Johnson, Table of n, a(n) for n = 1..200
EXAMPLE
The first terms of this sequence correspond to the following elements of A136113:
P_5 = P_7 - P_5,
P_23 = P_24 - P_7,
P_51 = P_66 - P_42,
P_71 = P_74 - P_21,
P_72 = P_80 - P_35,
P_99 = P_104 - P_32,
P_123 = P_144 - P_75,
P_239 = P_249 - P_70,
P_263 = P_274 - P_77,
P_311 = P_324 - P_91,
P_359 = P_374 - P_10.
PROG
(PARI) P(n)=n*(3*n-1)/2
isPent(t)=P(sqrtint((t*2)\3)+1)==t
{for( i=1, 999, for( j=1, i-1, isPent(P(i)+P(j))|next; for( k=i+1, (P(i)-1)\3, isPent(P(i)+P(k))&next(3)); print1(i", "); next(2)))}
CROSSREFS
KEYWORD
nonn
AUTHOR
R. J. Mathar and M. F. Hasler, Feb 07 2008
STATUS
approved