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Numbers k such that k^2 has an odd number of digits and the middle digit is 5.
3

%I #7 Oct 03 2023 18:47:36

%S 16,112,150,163,166,169,172,186,211,218,227,242,244,246,248,250,252,

%T 254,256,258,273,282,289,314,1032,1037,1042,1056,1061,1075,1084,1089,

%U 1098,1107,1116,1125,1134,1138,1147,1160,1173,1177,1194,1198,1219,1223,1227

%N Numbers k such that k^2 has an odd number of digits and the middle digit is 5.

%H Lars Blomberg, <a href="/A279425/b279425.txt">Table of n, a(n) for n = 1..10000</a>

%e 16^2 = 2(5)6, 218^2 = 47(5)24, 1075^2 = 115(5)625.

%t Select[Range[1227], OddQ[len=Length[IntegerDigits[#^2]]]&&Part[IntegerDigits[#^2], (len+1)/2]==5 &] (* _Stefano Spezia_, Oct 03 2023 *)

%Y Cf. A279420-A279424, A279426-A279429.

%K nonn,base,easy

%O 1,1

%A _Lars Blomberg_, Dec 22 2016