

A117284


Numbers k for which the cototient kphi(k) is a triangular number.


7



1, 2, 3, 5, 7, 9, 10, 11, 13, 17, 19, 23, 29, 31, 37, 39, 41, 43, 45, 47, 52, 53, 54, 55, 57, 59, 61, 67, 68, 71, 73, 79, 83, 85, 89, 90, 97, 101, 103, 107, 109, 113, 114, 117, 127, 129, 131, 137, 139, 149, 151, 157, 159, 163, 167, 168, 173, 175, 179, 181, 191, 193
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OFFSET

1,2


LINKS

Amiram Eldar, Table of n, a(n) for n = 1..10000 (terms 1..1001 from Harvey P. Dale)


EXAMPLE

39 is in the sequence because 39phi(39) = 15, which is a triangular number.


MATHEMATICA

Select[Range[200], OddQ[Sqrt[8(#EulerPhi[#])+1]]&] (* Harvey P. Dale, Jun 29 2015 *)


PROG

(PARI) isok(n) = ispolygonal(n  eulerphi(n), 3); \\ Michel Marcus, Feb 26 2014


CROSSREFS

Cf. A000010, A000217, A051953.
Cf. A117283, A117285, A117286, A117287, A117288, A117289.
Sequence in context: A302403 A096738 A167857 * A137377 A274793 A339238
Adjacent sequences: A117281 A117282 A117283 * A117285 A117286 A117287


KEYWORD

nonn


AUTHOR

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


EXTENSIONS

Offset corrected by Amiram Eldar, Mar 23 2021


STATUS

approved



