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%I #10 Jul 22 2021 07:50:55
%S 25721,28836,32129,35600,39249,43076,47081,51264,55625,60164,64881,
%T 69776,74849,80100,85529,91136,96921,102884,109025,115344,121841,
%U 128516,135369,142400,149609,156996,164561,172304,180225,188324,196601,205056
%N Numbers k such that the concatenation of k with 4*k gives a square.
%C Not the same as P(n) = 89*n^2 + 2848*n + 22784: P(n) = a(n) for a < 37, but a(37) = 3698225. - _Charles R Greathouse IV_, Jul 28 2010
%e 25721_102884 = 160378^2.
%t Select[Range[250000],IntegerQ[Sqrt[#*10^IntegerLength[4*#]+4*#]]&] (* _Harvey P. Dale_, Jan 04 2019 *)
%Y Cf. A102567, A106497, A115527 - A115556.
%K nonn,base
%O 1,1
%A _Giovanni Resta_, Jan 25 2006
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