OFFSET
1,1
COMMENTS
Subsequence of A103826.
Similar to A187073, but considering unitary divisors, not prime divisors.
The odd terms of the sequence are: (1) the terms of A005383 (numbers n such that both n and (n+1)/2 are primes) and (2) the terms of A192618 (prime powers p^k with even exponents k>0 such that (1+p^k)/2 is prime).
[Note that A034448(n) and A034444(n) are multiplicative, so the arithmetic mean A034448(n)/A034444(n) is multiplicative with a(p^e) = (1+p^e)/2.]
The even terms of the sequence are 6, 12, 48, 768, 196608,... (no others < 10^10) with formula n = 3*2^(2^(k-1)) and averages 3, 5, 17, 257, 65537, ... (Fermat numbers, A000215).
LINKS
Klaus Brockhaus, Table of n, a(n) for n = 1..10000
A. Roldan Martinez, Numeros y hoja de calculo
EXAMPLE
48 has unitary divisors 1, 3, 16, 48 and (1+3+16+48)/4 = 17 is prime, therefore 48 is in the sequence.
PROG
(Magma) UnitaryDivisors:=func< n | [ d: d in Divisors(n) | Gcd(d, n div d) eq 1 ] >; [ n: n in [1..2500] | IsPrime(k) and s mod #U eq 0 where k is s div #U where s is &+U where U is UnitaryDivisors(n) ]; // Klaus Brockhaus, Jul 09 2011
(PARI) usigma(n)= {local(f, u=1); f=factor(n); for(i=1, matsize(f)[1], u*=(1+ f[i, 1]^f[i, 2])); return(u)}
ud(n)= {local (f, u); f=factor(n); u=2^(matsize(f)[1]); return(u) }
{ for (n=2, 10^4, c=usigma(n)/ud(n); if (c==truncate(c), if(isprime(c), print1(n, ", ")))) }
// Antonio Roldán, Oct 08 2012
CROSSREFS
KEYWORD
nonn
AUTHOR
Antonio Roldán, Jul 04 2011
STATUS
approved