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A115905 Numbers n such that n+phi(n) is a triangular number. 4
2, 4, 9, 11, 20, 23, 25, 27, 49, 53, 92, 93, 105, 121, 127, 147, 150, 163, 169, 220, 228, 231, 233, 281, 286, 289, 294, 332, 361, 431, 474, 529, 541, 543, 548, 613, 660, 675, 710, 726, 743, 825, 827, 841, 888, 891, 961, 977, 993, 1014, 1028, 1265, 1310, 1369 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

LINKS

Table of n, a(n) for n=1..54.

EXAMPLE

121+phi(121)=231=T(21).

MATHEMATICA

Select[Range[1500], OddQ[Sqrt[8(#+EulerPhi[#])+1]]&] (* Harvey P. Dale, May 11 2015 *)

PROG

(PARI) isok(n) = ispolygonal(n + eulerphi(n), 3); \\ Michel Marcus, Jan 25 2014

CROSSREFS

Cf. A069066, A115908, A115910.

Sequence in context: A186699 A093859 A266257 * A292769 A307997 A096134

Adjacent sequences:  A115902 A115903 A115904 * A115906 A115907 A115908

KEYWORD

nonn

AUTHOR

Giovanni Resta, Feb 06 2006

STATUS

approved

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Last modified March 30 11:16 EDT 2020. Contains 333125 sequences. (Running on oeis4.)