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Search: id:A136115
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| A136115 |
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Index m of least pentagonal number P(m) > P(n) such that P(m)+P(n) is again a pentagonal number, 0 if no such m exists. |
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+0
6
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| 0, 0, 0, 7, 0, 0, 23, 0, 0, 48, 0, 22, 82, 47, 0, 125, 26, 0, 22, 37, 71, 238, 0, 0, 26, 166, 0, 52, 207, 147, 117, 99, 87, 572, 72, 67, 63, 357, 57, 110, 416, 51, 917, 82, 47, 1050, 217, 380, 167, 246, 0, 97, 697, 0, 374, 191, 537, 1672, 152, 112, 136, 380, 215, 2037, 68
(list; graph; listen)
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OFFSET
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1,4
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FORMULA
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a(n)=0 iff n is in A136112 iff A000326(n) is in A136113.
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EXAMPLE
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a(1..3)=0 since P(1),P(2),P(3) cannot be written as difference of 2 other pentagonal numbers > 0.
a(4)=7 since P(7)=70 is the least pentagonal number > P(4)=22 such that their sum is again a pentagonal number, P(8).
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PROGRAM
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(PARI) P(n)=n*(3*n-1)>>1 /* a.k.a. A000326 */ /* newline */ isPent(t)=P(sqrtint(t<<1\3)+1)==t /* newline */ for(i=1, 99, for(j=i+1, (P(i)-1)\3, isPent(P(i)+P(j))&print1(j", ")|next(2)); print1(0", "))
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CROSSREFS
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Cf. A000326, A136112-A136118.
Adjacent sequences: A136112 A136113 A136114 this_sequence A136116 A136117 A136118
Sequence in context: A132712 A021592 A094898 this_sequence A061846 A121570 A101031
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KEYWORD
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nonn
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AUTHOR
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M. F. Hasler (Maximilian.Hasler(AT)gmail.com), Dec 15 2007
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