A072963 - OEIS

A072963

In prime factorization of n replace all single (i.e., non-twin) primes with 1.

1, 1, 3, 1, 5, 3, 7, 1, 9, 5, 11, 3, 13, 7, 15, 1, 17, 9, 19, 5, 21, 11, 1, 3, 25, 13, 27, 7, 29, 15, 31, 1, 33, 17, 35, 9, 1, 19, 39, 5, 41, 21, 43, 11, 45, 1, 1, 3, 49, 25, 51, 13, 1, 27, 55, 7, 57, 29, 59, 15, 61, 31, 63, 1, 65, 33, 1, 17, 3, 35, 71, 9, 73, 1, 75, 19, 77, 39, 1, 5, 81

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OFFSET

1,3

COMMENTS

a(a(n)) = a(n).

LINKS

Amiram Eldar, Table of n, a(n) for n = 1..10000

FORMULA

Multiplicative with a(p) = (if p+2 or p-2 is prime then p else 1), p prime.

EXAMPLE

a(92) = a(2*2*23) = 1; a(93) = a(3*31) = 3*31 = 93.

MATHEMATICA