OFFSET
0,1
COMMENTS
If there are only five Fermat primes, then a(n) = 2^(n-30) * 99852066765 for n > 31. - T. D. Noe, Jun 21 2012
LINKS
T. D. Noe, Table of n, a(n) for n = 0..100
EXAMPLE
For n=6, 2^n=64; the solutions of phi(x)=64 are {85,128,136,160,170,192,204,240}, whose sum is a(6)=1315.
MATHEMATICA
phiinv[n_, pl_] := Module[{i, p, e, pe, val}, If[pl=={}, Return[If[n==1, {1}, {}]]]; val={}; p=Last[pl]; For[e=0; pe=1, e==0||Mod[n, (p-1)pe/p]==0, e++; pe*=p, val=Join[val, pe*phiinv[If[e==0, n, n*p/pe/(p-1)], Drop[pl, -1]]]]; Sort[val]]; phiinv[n_] := phiinv[n, Select[1+Divisors[n], PrimeQ]]; Table[Plus@@phiinv[2^n], {n, 0, 30}] (* phiinv[n, pl] = list of x with phi(x)=n and all prime divisors of x in list pl. phiinv[n] = list of x with phi(x)=n *)
CROSSREFS
KEYWORD
nonn
AUTHOR
Labos Elemer, Nov 30 2000
EXTENSIONS
Edited by Dean Hickerson, Jan 25 2002
a(28)-a(29) from Donovan Johnson, Oct 22 2011
STATUS
approved