A292238 a(n) = gpf(lpf(2^prime(n) - 1) - 1) where prime(n) is the n-th prime.
2, 3, 5, 7, 11, 13, 257, 73, 23, 29, 331, 37, 163, 43, 47, 53, 61, 1321, 2677, 1609, 73, 79, 83, 2931542417, 97, 278557, 599, 28059810762433, 31981, 113, 77158673929, 131, 27977333, 457, 37888318897441, 151, 4523, 461, 541, 173, 179, 181, 191, 587, 197, 3690437
Offset: 1
Keywords
Programs
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PARI
a(n) = vecmax(factor(vecmax(factor(2^prime(n)-1)[, 1])-1)[, 1]);
Formula
For p in A291691, let pp be such that p = prime(pp), then a(pp) = p.