For n>1; a(n) is a multiple of a Fermat prime (
A019434). Subsequence of
A071593.
For all divisors d_k of a(n) we have
A000120(d_k) = k.
Subsequence of known numbers with k divisors:
for k = 2: 3, 5, 17, 257, 65537, ... - Fermat primes (
A019434);
for k = 3: 25, 289, 66049, 4295098369, ... - some square of Fermat prime;
for k = 4: 39, 57, 201, 291, 323, 393, 579, 2307, 12297, 36867, 98313, 196617, 197633, 786441, 2359299, 805306377, 3221225481, 4295229443, 9663676419, 618475290627, 19791209299971, ... - some products of two distinct primes p*q, where p is a Fermat prime (
A019434) and q is a term of sequence
A081091, see (Magma) - Set(Sort([n*m: n in [
A019434(n)], m in [
A081091(m)] | n lt m and &+Intseq(n, 2) eq 2 and &+Intseq(m, 2) eq 3 and &+Intseq(n*m, 2) eq 4]));
for k = 6: 1083 - the only number with this property < 10^7;
for k = 8: 7955, 8815, 9399, 12909, 13737, 40521, 43797, 50349, 66291, 66531, 68457, 80457, 160329, 230691, 299559, 599079, 922179, 1278537, 2396199, 2556489, ...; see (Magma) - Set(Sort([n: n in [1..1000000] | [&+Intseq(d, 2): d in Divisors(n)] eq [1,2,3,4,5,6,7,8]])).
Conjectures: 1) Sequence is infinite. 2) 8 is the maximal value of k for numbers with this property.
Numbers 805306377, 3221225481, 4295098369, 4295229443, 9663676419, 618475290627 and 19791209299971 are also terms of this sequence.
Sequence of the smallest numbers n with k divisors having these properties for k >= 1 or 0 if no solution exists or has been found: 1, 3, 25, 39, 0, 1083, 0, 7955, ...; a(5) = a(7) = 0 if there are only 5 Fermat primes. Conjecture: a(k) = 0 for k > 8.
