[seqfan] Sophie Germain pentagonal numbers

Jonathan Post jvospost3 at gmail.com
Sat Mar 13 00:11:32 CET 2010

The pentagonal number analogue of
A124174  	 Sophie Germain triangular numbers tr: 2*tr+1 is also a
triangular number.

2*(A000326(j))+1 = A000326(k)

begins (denoting P(n) = n-th pentagonal number):
2P(0)+1 = 2*0+1 = 1 = P(1)
2P(75)+1 = 2*8400+1 = 16801 = P(106).

What are the next such values, which seem to be obtainable through a
Pell's equation?

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