A273179 - OEIS

%I #13 Dec 12 2021 22:34:54

%S 119,161,217,238,287,322,329,391,434,497,511,527,553,574,623,658,679,

%T 697,713,721,782,791,799,833,889,943,959,994,1022,1054,1057,1081,1106,

%U 1127,1169,1207,1241,1246,1271,1337,1343,1351,1358,1393,1394,1426,1442,1457,1513

%N Numbers k for which 2 has exactly four square roots mod k.

%C Sequence is fairly regular; 437 numbers below 1000 have 4 square roots of 2 mod k; 4796 below 10^4; 47276 below 10^5; and 452172 below 10^6.

%e 2 has square roots 11, 45, 74, 108 mod 119; e.g., 11^2 == 2 (mod 119).

%t Select[Range[3000], Length@PowerModList[2, 1/2, #] == 4 &]

%Y Subsequence of A057126 (Numbers n such that 2 is a square mod n).

%K nonn

%O 1,1

%A _Dale Taylor_, May 17 2016