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A117287
Numbers k for which the cototient k-phi(k) is a heptagonal number.
7
1, 2, 3, 5, 7, 11, 13, 15, 17, 19, 23, 29, 31, 34, 37, 41, 43, 47, 49, 53, 59, 61, 67, 71, 73, 79, 83, 89, 97, 101, 103, 107, 109, 113, 127, 131, 137, 139, 149, 151, 157, 159, 163, 167, 173, 175, 179, 181, 189, 191, 193, 196, 197, 199, 208, 211, 223, 227, 229, 233
OFFSET
1,2
LINKS
EXAMPLE
49 is in the sequence because 49-phi(49) = 7, which is a heptagonal number.
MATHEMATICA
hnQ[n_]:=Module[{c=n-EulerPhi[n]}, IntegerQ[(3+Sqrt[9+40c])/10]]; Join[{1}, Select[Range[0, 250], hnQ]] (* Harvey P. Dale, Mar 26 2011 *)
PROG
(PARI) isok(n) = ispolygonal(n - eulerphi(n), 7); \\ Michel Marcus, Feb 26 2014
KEYWORD
nonn
AUTHOR
Luc Stevens (lms022(AT)yahoo.com), Apr 23 2006
EXTENSIONS
Offset corrected by Amiram Eldar, Mar 23 2021
STATUS
approved