login
Numbers k such that k^2 with last digit deleted is a prime.
1

%I #22 Sep 08 2022 08:46:19

%S 5,6,14,26,44,46,56,64,74,76,86,94,106,146,154,164,206,226,236,244,

%T 254,256,274,286,296,304,314,326,344,346,364,424,436,446,454,464,506,

%U 524,536,596,614,664,674,676,686,694,706,764,776,796,826,844,854,874,944,946

%N Numbers k such that k^2 with last digit deleted is a prime.

%H Marius A. Burtea, <a href="/A289190/b289190.txt">Table of n, a(n) for n = 1..545</a>

%e 14 is in the sequence because 14^2 = 196; deleting the last digit gives 19 which is prime.

%e 26 is in the sequence because 26^2 = 676; deleting the last digit gives 67 which is prime.

%p select(n -> isprime(floor(n^2/10)),[$1..2000]);

%t fQ[n_] := PrimeQ@Quotient[n^2, 10]; Select[Range[1, 2000], fQ]

%o (Magma) [n : n in [1 .. 2000] | IsPrime (Floor (n^2/10))];

%o (PARI) isok(n) = isprime(n^2\10); \\ _Michel Marcus_, Jul 02 2017

%Y Cf. A024770, A031149, A137812, A227916, A227919, A258032.

%K nonn,base

%O 1,1

%A _K. D. Bajpai_, Jun 27 2017