OFFSET
1,2
LINKS
Robert Israel, Table of n, a(n) for n = 1..10000
EXAMPLE
44 is in the sequence because 44-phi(44) = 24, which is an enneagonal number.
MAPLE
N:= 1000: # to get all terms <= N
enneagonal:= [seq(n*(7*n-5)/2, n=0..floor((sqrt(25+56*N)+5)/14))]:
select(t -> member(t - numtheory:-phi(t), enneagonal), [$1..N]); # Robert Israel, Mar 30 2018
MATHEMATICA
ennQ[n_] := n == 0 || IntegerQ[(Sqrt[56*n + 25] + 5)/14]; Select[Range[250], ennQ[# - EulerPhi[#]] &] (* Amiram Eldar, Mar 23 2021 *)
PROG
(PARI) isok(n) = ispolygonal(n - eulerphi(n), 9); \\ Michel Marcus, Feb 26 2014
CROSSREFS
KEYWORD
nonn
AUTHOR
Luc Stevens (lms022(AT)yahoo.com), Apr 23 2006
EXTENSIONS
Offset changed to 1 by Robert Israel, Mar 30 2018
STATUS
approved