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 A277401 Positive integers n such that 7^n == 2 (mod n). 7
 1, 5, 143, 1133, 2171, 8567, 16805, 208091, 1887043, 517295383, 878436591673 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS All terms are odd. No other terms below 10^15. Some larger terms: 181204957971619289, 21305718571846184078167, 157*(7^157-2)/1355 (132 digits). - Max Alekseyev, Oct 18 2016 LINKS FORMULA A066438(a(n)) = 2 for n > 1. EXAMPLE 7 == 2 mod 1, so 1 is a term; 16807 == 2 mod 5, so 5 is a term. MATHEMATICA Join[{1}, Select[Range[5173*10^5], PowerMod[7, #, #]==2&]] (* The program will generate the first 10 terms of the sequence; it would take a very long time to generate the 11th term. *) (* Harvey P. Dale, Apr 15 2020 *) PROG (PARI) isok(n) = Mod(7, n)^n == 2; \\ Michel Marcus, Oct 13 2016 CROSSREFS Cf. A066438. Cf. Solutions to 7^n == k (mod n): A277371 (k=-3), A277370 (k=-2), A015954 (k=-1), A067947 (k=1), this sequence (k=2), A277554 (k=3). Cf. Solutions to b^n == 2 (mod n): A015919 (b=2), A276671 (b=3), A130421 (b=4), A124246 (b=5), this sequence (b=7), A116622 (b=13). Sequence in context: A006269 A066264 A171776 * A208874 A222289 A134503 Adjacent sequences:  A277398 A277399 A277400 * A277402 A277403 A277404 KEYWORD nonn,more AUTHOR Seiichi Manyama, Oct 13 2016 EXTENSIONS a(10) from Michel Marcus, Oct 13 2016 a(11) from Max Alekseyev, Oct 18 2016 STATUS approved

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Last modified June 1 04:57 EDT 2020. Contains 334758 sequences. (Running on oeis4.)