Year-end appeal: Please make a donation to the OEIS Foundation to support ongoing development and maintenance of the OEIS. We are now in our 61st year, we have over 378,000 sequences, and we’ve reached 11,000 citations (which often say “discovered thanks to the OEIS”).
%I #24 Feb 26 2022 16:34:54
%S 1,2,14,1022,20066,80519,107663,485918,1284113,1510313,2531678,
%T 3677198,3933023,4557713,8277458,8893262,21122318,24849833,26358638,
%U 39852014,42448478,71871113,76712318,80646143,98058097,104832833,106694033,131492498,144322478,146987033,164360606,168204191,175126478,176647378,188997463,196705598
%N Numbers k such that 4^k == 2 (mod k).
%C Some terms above 10^15: 26435035805519327, 158975398896178078, 64044049390757098943, 1063423126446412987081943, 1091220919655042176978844783, 81074850280355100090334498663, 6317483763950169936179578094903, 5672799393875320397186007124651847, 170923900137537174138295268515194974, 195746953975871672436191077726091399305155458, 325665752547333314939363628501536024940097079718953, 953533053776414279913696071891872697927468471633033, 85791212788381063775490416118630897060666265030605503, 334519297382630382793758729321508383611586565722054114034741260213364710519401967713. - _Max Alekseyev_, Jun 18 2014
%H Max Alekseyev, <a href="/A130421/b130421.txt">Table of n, a(n) for n = 1..2617</a> (all terms below 10^15)
%t Join[{1,2},Select[Range[107000000],PowerMod[4,#,#]==2&]] (* _Harvey P. Dale_, Jun 13 2013 *)
%Y Cf. A006935 (odd terms times 2), A130422, A347906 (odd terms), A347908 (even terms).
%K nonn
%O 1,2
%A _Jon E. Schoenfield_, May 26 2007
%E Terms a(28) onward from _Max Alekseyev_, Jun 18 2014
%E b-file corrected by _Max Alekseyev_, Oct 09 2016