OFFSET
1,2
COMMENTS
Is a(n+1)/a(n) ~ 4/3 for large n? - David A. Corneth, Mar 21 2015
LINKS
Hiroaki Yamanouchi, Table of n, a(n) for n = 1..50
Paul Pollack and Carl Pomerance, Prime-Perfect Numbers, INTEGERS, Electronic J. of Combinatorial Number Theory, Vol. 12a, Paper A14, 2012.
EXAMPLE
The first terms of A055744 are 1, 4, 8, 16, 18, 32, among which 18 is the only integer that is not a power of 2. Thus this sequence starts with 1, 2, 3, 4, 6.
MATHEMATICA
t = Select[Range@ 1000000, First /@ FactorInteger@# == First /@ FactorInteger@ EulerPhi@ # &]; f[n_] := Block[{pf = FactorInteger@ n, p2}, p2 = First@ First@ pf; If[Length@ pf == 1 && First@ First@ pf == 2, Last@ First@ pf, 0]]; {1}~Join~Flatten@ Position[f /@ t, n_ /; n > 0] (* Michael De Vlieger, Mar 21 2015 *)
PROG
(PARI) lista(nn) = {nb = 0; for (n=1, nn, if (factor(n)[, 1]==factor(eulerphi(n))[, 1], nb++; if (n == 2^valuation(n, 2), print1(nb, ", ")); ); ); }
(Haskell)
a256248 n = a256248_list !! (n-1)
a256248_list = filter ((== 1) . a209229 . a055744) [1..]
-- Reinhard Zumkeller, Jun 01 2015
CROSSREFS
KEYWORD
nonn
AUTHOR
Michel Marcus, Mar 20 2015
EXTENSIONS
a(30)-a(45) from Hiroaki Yamanouchi, Mar 31 2015
STATUS
approved