Somer pseudoprime

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A Somer pseudoprime is an odd composite number based on another number. They can be obtained by this process from a number D:

[(D^2) - (2D -1 ) + 1] if D is odd

The first nine Somer pseudoprimes thus are

1, 3, 9, 15, 25, 35, 49 and 63.

These numbers are always odd, and one notices the digit pattern 1-3-9-5-5-5-9-3-1. This type of pseudoprime is often confused with Somer-Lucas pseudoprimes, which also have a number P and are formed by a Lucas sequence. The symbol for a Somer number is [S^Q].

This is sequence [A085046] of the On-Line Encyclopedia of Integer Sequences.

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