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 A136112 Indices of pentagonal numbers > 0 which are not the difference of two larger pentagonal numbers. 12

%I

%S 1,2,3,5,6,8,9,11,15,18,23,24,27,51,54,71,72,81,96,99,123,128,135,162,

%T 216,239,243,263,288,303,311,359,384,423,459,479,486,519,591,599,639,

%U 648,683,699,729,743,783,863,864,879,891,911,1031,1103,1151,1215,1431

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

%H Donovan Johnson, <a href="/A136112/b136112.txt">Table of n, a(n) for n = 1..500</a>

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

%e a(1..3)=1,2,3 since P(1),P(2),P(3) cannot be written as difference of 2 other pentagonal numbers.

%e P(4)=22=P(8)-P(7), therefore 4 is not in this sequence.

%o (PARI) P(n)=n*(3*n-1)>>1

%o isPent(t)=P(sqrtint((t<<1)\3)+1)==t

%o for( i=1,999,for( j=i+1,(P(i)-1)\3, isPent(P(i)+P(j))&next(2)); print1(i","))

%Y Cf. A000326, A136113-A136118.

%K nonn

%O 1,2

%A _M. F. Hasler_, Dec 15 2007

%E Definition corrected, thanks to a remark from R. J. Mathar, Feb 01 2008

%E More terms from _Donovan Johnson_, Apr 22 2008

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Last modified September 26 05:53 EDT 2022. Contains 356986 sequences. (Running on oeis4.)