

A117289


Numbers n for which the cototient nphi(n) is an enneagonal number (A001106).


1



1, 2, 3, 5, 7, 11, 13, 17, 19, 21, 23, 27, 29, 31, 36, 37, 40, 41, 43, 44, 46, 47, 53, 59, 61, 66, 67, 70, 71, 73, 79, 83, 89, 97, 101, 103, 107, 109, 113, 127, 131, 137, 139, 149, 151, 157, 163, 167, 173, 179, 181, 191, 193, 197, 199, 207, 211, 219, 223, 227, 229, 231
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OFFSET

1,2


LINKS

Robert Israel, Table of n, a(n) for n = 1..10000


EXAMPLE

44 is in the sequence because 44phi(44)=24, which is an enneagonal number.


MAPLE

N:= 1000: # to get all terms <= N
enneagonal:= [seq(n*(7*n5)/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


PROG

(PARI) isok(n) = ispolygonal(n  eulerphi(n), 9); \\ Michel Marcus, Feb 26 2014


CROSSREFS

Cf. A001106, A051953.
Sequence in context: A152073 A331046 A230606 * A328513 A318719 A106317
Adjacent sequences: A117286 A117287 A117288 * A117290 A117291 A117292


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



