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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 (list; graph; refs; listen; history; text; internal format)
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: 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

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Last modified March 19 13:29 EDT 2019. Contains 321330 sequences. (Running on oeis4.)