1,1

Subsequence of A227993.

The corresponding primes are 2, 3, 2, 5, 13, 5, 5, 7, 3, 11, 41, 11, 89, 2, 5, 37, 19, 13, 53, 37, ...

a(n) is a power of 2 for n = 1, 2, 4, 8, 12, 18, ... with the corresponding primes 2, 3, 5, 7, 11, 13, ...

a(n) is a perfect square for n = 1, 2, 3, 4, 8, 12, 14, 17, 18, ... with the corresponding primes 2, 3, 2, 5, 7, 11, 2, 19, 13, ...

Table of n, a(n) for n=1..20.

64 is in the sequence because the divisors of 64 are {1, 2, 4, 8, 16, 32, 64} and 1/2 + 2/4 + 4/8 + 8/16 + 16/32 + 32/64 = 3 is prime.

Do[s=0; Do[s=s+Divisors[n][[i]]/Divisors[n][[i+1]], {i, 1, Length[Divisors[n]]-1}]; If[PrimeQ[s]&&!PrimeQ[n], Print[n]], {n, 10^6}]

Cf. A227993.

Sequence in context: A175209 A141840 A203281 * A065404 A327496 A330824

Adjacent sequences: A255573 A255574 A255575 * A255577 A255578 A255579

nonn,more

Michel Lagneau, Feb 25 2015

approved