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 A083255 Odd composite numbers k such that cototient(k) - phi(k) = k - 2*phi(k) is an odd prime. 7
 165, 195, 5187, 5865, 7395, 10005, 15045, 16215, 21165, 22695, 27285, 37995, 42585, 44115, 50235, 57885, 59415, 60945, 64005, 310845, 346035, 347565, 486795, 635205, 707115, 890445, 979455, 994755, 1049835, 1070535, 1078815, 1083585, 1121745 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS Quite a number of terms are divisible by 3*5*17 = 255. LINKS Amiram Eldar, Table of n, a(n) for n = 1..10000 (terms 1..369 from R. J. Mathar) EXAMPLE m = 17425605 = 3*5*23*53*953 is a term since cototient(m) - phi(m) = 9712901 - 8712704 = 197 is an odd prime. MATHEMATICA Do[s=EulerPhi[n]; c=n-s; If[Greater[c, s]&&PrimeQ[c-s]&&OddQ[c-s]&&!PrimeQ[n], Print[{n, c-s, n/255}]], {n, 1, 10000000}] CROSSREFS Cf. A000010, A051953, A036798, A067800, A083254. Sequence in context: A261758 A252725 A168355 * A259283 A319502 A270175 Adjacent sequences: A083252 A083253 A083254 * A083256 A083257 A083258 KEYWORD nonn AUTHOR Labos Elemer, May 08 2003 STATUS approved

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Last modified November 29 22:05 EST 2023. Contains 367452 sequences. (Running on oeis4.)