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 A015949 Numbers k such that k | 3^k + 1. 23
 1, 2, 10, 50, 250, 1250, 5050, 6250, 11810, 25250, 31250, 59050, 126250, 156250, 295250, 510050, 631250, 750250, 781250, 1476250, 2125250, 2550250, 3156250, 3751250, 3906250, 5964050, 7381250, 10626250, 12751250, 13947610, 15781250 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS a(n) mod 20 = 10 for n >= 3. - G. C. Greubel, Nov 05 2018 This sequence is infinite, because for n > 1, 3^a(n) + 1 is in this sequence. - Jinyuan Wang, Nov 06 2018 For the provided data, if k is a term then p*k is a term where p is an odd divisor of k. - David A. Corneth, Nov 06 2018 LINKS Giovanni Resta, Table of n, a(n) for n = 1..180 (first 100 terms from G. C. Greubel) MATHEMATICA Do[If[PowerMod[3, n, n] + 1 == n, Print[n]], {n, 1, 10^7}] (* Jinyuan Wang, Nov 01 2018 *) PROG (PARI) for(n=1, 10^7, if(Mod(3, n)^n==-1, print1(n, ", "))) \\ Jinyuan Wang, Nov 01 2018 (MAGMA) [n: n in [1..2*10^7]| Modexp(3, n, n)+1 eq n]; // Vincenzo Librandi, Nov 01 2018 CROSSREFS Cf. A034472 (3^n+1). Cf. A006521 (k | 2^k + 1), A015950 (k | 4^k + 1), A015951 (k | 5^k + 1). Column k=3 of A333429. Sequence in context: A180266 A015945 A015954 * A020699 A020729 A110170 Adjacent sequences:  A015946 A015947 A015948 * A015950 A015951 A015952 KEYWORD nonn AUTHOR EXTENSIONS Corrected by David W. Wilson STATUS approved

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Last modified May 17 18:44 EDT 2021. Contains 343987 sequences. (Running on oeis4.)