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a(n) = least k such that the remainder when 19^k is divided by k is n.
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%I #17 Aug 01 2015 06:04:50

%S 2,17,358,5,7,13,118,11,22,207,14,6683,21,1055,221,6843,86,39959,23,

%T 559,34,129,26,25,51,799,334,33,166,47427581,1537,901,68,39,326,87169,

%U 44,161,46,3509,341,529,106,1098179,158,657,314,49621349,75,143,62,749,116

%N a(n) = least k such that the remainder when 19^k is divided by k is n.

%C a(447) = 7987803178, a(660) = 11147676413, a(923) = 6246715274. - _Daniel Morel_, Jun 08 2010

%C a(216) = 21686254249, a(296) = 40778012377, a(386) = 15891209603, a(582) = 46530896443, a(638) = 15297472657, a(736) = 45211411479, a(872) = 106458212591. - _Daniel Morel_, Oct 15 2010

%H Robert G. Wilson v, <a href="/A128159/a128159.txt">Table of n, a(n) for n = 1..10000 with -1 for large entries where a(n) has not yet been found</a>

%t t = Table[0, {10000} ]; k = 1; While[ k < 3100000000, a = PowerMod[19, k, k]; If[a < 10001 && t[[a]] == 0, t[[a]] = k; Print[{a, k}]]; k++ ]; t (* _Robert G. Wilson v_, Aug 04 2009 *)

%t clk=Compile[{{n,_Integer}},{k=1};While[PowerMod[19,k,k]!=n,k++];k]; Array[ clk,55] (* _Harvey P. Dale_, May 10 2014 *)

%Y Cf. A036236, A078457, A119678, A119679, A127816, A119715, A119714, A127817, A127818, A127819, A127820, A127821, A128154, A128155, A128156, A128157, A128158, A128160.

%Y Cf. A128149, A128150.

%K hard,nonn

%O 1,1

%A _Alexander Adamchuk_, Feb 16 2007

%E More terms from _Ryan Propper_, Mar 24 2007

%E More terms from _Robert G. Wilson v_, Aug 04 2009