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A136112
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Indices of pentagonal numbers > 0 which are not the difference of two larger pentagonal numbers.
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12
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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
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1,2
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LINKS
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Donovan Johnson, Table of n, a(n) for n = 1..500
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FORMULA
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A number m is in this sequence iff A136114(m) = 0 iff A136115(m) = 0
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EXAMPLE
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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.
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PROG
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(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", "))
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CROSSREFS
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Cf. A000326, A136113-A136118.
Sequence in context: A027563 A219729 A000534 * A127936 A096276 A075725
Adjacent sequences: A136109 A136110 A136111 * A136113 A136114 A136115
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KEYWORD
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nonn
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AUTHOR
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M. F. Hasler, Dec 15 2007
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EXTENSIONS
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Definition corrected, thanks to a remark from R. J. Mathar, Feb 01 2008
More terms from Donovan Johnson, Apr 22 2008
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STATUS
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approved
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