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A117285 Numbers n for which the cototient n-phi(n) is a pentagonal number. 0
1, 2, 3, 5, 7, 11, 13, 17, 18, 19, 20, 22, 23, 25, 29, 30, 31, 37, 41, 43, 47, 53, 59, 61, 67, 71, 73, 75, 79, 83, 89, 97, 101, 102, 103, 107, 109, 110, 113, 127, 131, 132, 137, 139, 140, 149, 151, 155, 157, 163, 167, 173, 179, 181, 191, 193, 197, 199, 203, 211, 223 (list; graph; refs; listen; history; text; internal format)
OFFSET

0,2

LINKS

Table of n, a(n) for n=0..60.

EXAMPLE

30 is in the sequence because 30-phi(30)=22, which is a pentagonal number.

PROG

(PARI) isok(n) = ispolygonal(n - eulerphi(n), 5); \\ Michel Marcus, Feb 26 2014

CROSSREFS

Cf. A000326, A051953.

Sequence in context: A171049 A171053 A171055 * A100546 A117094 A171057

Adjacent sequences:  A117282 A117283 A117284 * A117286 A117287 A117288

KEYWORD

nonn

AUTHOR

Luc Stevens (lms022(AT)yahoo.com), Apr 23 2006

STATUS

approved

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Last modified May 26 02:56 EDT 2020. Contains 334613 sequences. (Running on oeis4.)