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 A115905 Numbers k such that k + phi(k) is a triangular number. 5
 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 Amiram Eldar, Table of n, a(n) for n = 1..10000 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. A000010, A000217, 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 June 1 03:55 EDT 2023. Contains 363068 sequences. (Running on oeis4.)