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%I #18 Sep 08 2022 08:46:16
%S 5,25,56,89,116,209,304,401,500,601,704,809,916,1025,1316,1609,1904,
%T 2201,2500,2801,3104,3409,3716,4025,4336,4649,4964,5281,5600,5921,
%U 6244,6569,6896,7225,7556,7889,8224,8561,8900,9241,9584,9929,10681
%N Numbers n such that the decimal number concat(2,n) is a square.
%C Elements are squares of integers in (sqrt(21), sqrt(30)) * sqrt(10)^k without the leading 2 elements for nonnegative k. - _David A. Corneth_, May 20 2016
%H Nathan Fox, <a href="/A273357/b273357.txt">Table of n, a(n) for n = 1..13085</a>
%e 56 is a member because 256 = 16^2 is a square.
%e 0 is not a member because 20 is not a square.
%p t1:=[];
%p for k from 1 to 20000 do
%p if issqr(k+2*10^length(k)) then t1:=[op(t1), k]; fi;
%p od;
%p t1;
%o (Magma) [n: n in [1..15000 ] | IsSquare(Seqint(Intseq(n) cat Intseq(2)))]; // _Marius A. Burtea_, Mar 21 2019
%Y Cf. A272671, A273358, A273359, A273360, A273361, A273362, A273363, A273364.
%K nonn,base
%O 1,1
%A _Nathan Fox_, _Brooke Logan_, and _N. J. A. Sloane_, May 20 2016
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