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 A117289 Numbers n for which the cototient n-phi(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 (list; graph; refs; listen; history; text; internal format)
 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 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

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Last modified January 22 19:39 EST 2020. Contains 331153 sequences. (Running on oeis4.)