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a(n) = least k such that the remainder when 5^k is divided by k is n.
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%I #29 Jun 23 2018 14:20:11

%S 2,3,22,4769,7,15853,114,9,28,35,14,1328467,68,111,1555,9569200211,76,

%T 2030227,49,21,299,1097122717,51,546707,26,27,121,529,596,3095,138,93,

%U 136,34723,45,589,198,87,18142961,595,292,319,318,117,55,20485243,91

%N a(n) = least k such that the remainder when 5^k is divided by k is n.

%C From _Alexander Adamchuk_, Jan 31 2007: (Start)

%C a(n) > n.

%C For numbers n such that a(n-1) = n, see A015951 except first term. (End)

%C a(58) <= 16860204577843069 from Joe K. Crump (joecr(AT)carolina.rr.com), Feb 06 2007

%H Robert G. Wilson v, <a href="/A119679/a119679.txt">Table of n, a(n) for n = 1..10000 with -1 for those entries where a(n) has not yet been found</a> [This file is now out of date - see the extension lines below. - _N. J. A. Sloane_, May 22 2010]

%t Do[k = 1; While[PowerMod[5, k, k] != n, k++ ]; Print[k], {n, 30}]

%t Table[0, {10000}]; k = 1; lst = {}; While[k < 5000000000, a = PowerMod[5, k, k]; If[ a < 10001 && t[[a]] == 0, t[[a]] = k; Print[{a,k}]]; k++ ]; t (* changed (to reflect the new limits) by _Robert G. Wilson v_, Jul 14 2009 *)

%Y Cf. A015951, A036236, A078457, A119678, A127816, A119715, A119714, A127817, A127818, A127819, A127820, A127821.

%K nonn

%O 1,1

%A _Ryan Propper_, Jun 12 2006

%E Revised by _Max Alekseyev_, Sep 25 2007

%E a(172) = 26598818717 = 23 * 593 * 1039 * 1877, a(288) = 9158745413 = 241 * 347 * 109519, a(518) = 33288260241 = 3 * 43 * 258048529, a(558) = 7722115807 = 7 * 157 * 7026493 from _Daniel Morel_, May 18 2010

%E a(848) = 6672480963 = 3 * 241 * 9228881 from _Daniel Morel_, May 26 2010

%E a(416) = 10545901269 from _Daniel Morel_, Jul 05 2010

%E a(948) = 146246024857 from _Daniel Morel_, Jul 12 2010

%E a(822) = 466661006683 from _Daniel Morel_, Aug 24 2010