OFFSET
1,1
COMMENTS
a(14) > 10^9.
EXAMPLE
MATHEMATICA
uphi[0] = 0; uphi[1] = 1; uphi[n_] := (Times @@ (Table[#[[1]]^#[[2]] - 1, {1}] & /@ FactorInteger[n]))[[1]]; ucot[n_] := n - uphi[n]; Select[Range[10^4], Plus @@ FixedPointList[ucot, #] == 2*# + 1 &]
CROSSREFS
KEYWORD
nonn,more
AUTHOR
Amiram Eldar, Mar 07 2020
STATUS
approved