

A136112


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


12



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
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OFFSET

1,2


LINKS

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


FORMULA

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


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*n1)>>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

Cf. A000326, A136113A136118.
Sequence in context: A027563 A219729 A000534 * A127936 A280771 A280744
Adjacent sequences: A136109 A136110 A136111 * A136113 A136114 A136115


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



