A136112 - OEIS

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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

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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*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

Cf. A000326, A136113-A136118.

Sequence in context: A219729 A335659 A000534 * A127936 A280771 A368363

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