A167042 - OEIS

%I #14 Mar 15 2024 02:24:19

%S 225,5625,22500,562500,2250000,6125625,19580625,56250000,225000000,

%T 612562500,1958062500,2562890625,5625000000,22500000000,51472265625,

%U 61256250000,195806250000,256289062500,562500000000,2250000000000,5147226562500,6125625000000

%N Squares that remain squares when prefixed with a 7.

%H Chai Wah Wu, <a href="/A167042/b167042.txt">Table of n, a(n) for n = 1..1000</a>

%e 225 = 15^2 is a square, and becomes the square 7225 = 85^2 if prefixed with a 7.

%t Select[Range[25*10^5]^2,IntegerQ[Sqrt[#+7*10^IntegerLength[#]]]&] (* _Harvey P. Dale_, Mar 02 2015 *)

%K nonn,base

%O 1,1

%A _Claudio Meller_, Oct 27 2009

%E Keyword:base added by _R. J. Mathar_, Oct 28 2009

%E More terms from _Chai Wah Wu_, Sep 11 2014