OFFSET
1,2
COMMENTS
55 is in the sequence because 55-phi(55) = 15, which is a hexagonal number.
LINKS
Amiram Eldar, Table of n, a(n) for n = 1..10000
MATHEMATICA
cthQ[n_]:=Module[{c=n-EulerPhi[n]}, IntegerQ[(Sqrt[1+8c]+1)/4]]; Join[{1}, Select[Range[250], cthQ]] (* Harvey P. Dale, Jan 16 2012 *)
PROG
(PARI) isok(n) = ispolygonal(n - eulerphi(n), 6); \\ Michel Marcus, Feb 26 2014
CROSSREFS
KEYWORD
nonn
AUTHOR
Luc Stevens (lms022(AT)yahoo.com), Apr 23 2006
EXTENSIONS
Corrected by T. D. Noe, Oct 25 2006
STATUS
approved