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A343724 a(n) is the smallest n-digit square with all digits even.

%I A343724 #29 Jun 02 2021 10:15:19
%S A343724 0,64,400,4624,26244,228484,2022084,20268004,202208400,2006860804,
%T A343724 20220840000,200084446864,2002004266084,20000286620224,
%U A343724 200080402620484,2000028662022400,20000086482842244,200002866202240000,2000008648284224400,20000246442286866064
%N A343724 a(n) is the smallest n-digit square with all digits even.
%C A343724 From _Robert G. Wilson v_, May 20 2021: (Start)
%C A343724 The square root of any term is == {0, 2, 8} (mod 10).
%C A343724 Other than 1 and 9, there are no squares which contain only odd digits.
%C A343724 (End)
%H A343724 Chai Wah Wu, <a href="/A343724/b343724.txt">Table of n, a(n) for n = 1..61</a> (terms for n = 1..40 from Robert G. Wilson v)
%t A343724 a[n_] := Block[{k = Floor[ Sqrt[10^n/5]]}, If[OddQ@k, k--]; While[ Union[ EvenQ[ IntegerDigits[ k^2]]] != {True}, k += 2]; k^2]; Array[ a, 20] (* _Robert G. Wilson v_, May 20 2021 *)
%o A343724 (Python 3.8+)
%o A343724 from math import isqrt
%o A343724 def A343724(n):
%o A343724     if n == 1: return 0
%o A343724     m = isqrt(2*10**(i-1))+1
%o A343724     m += m % 2
%o A343724     k = m**2
%o A343724     s = set('02468')
%o A343724     while not set(str(k)) <= s:
%o A343724         m += 2
%o A343724         k += 4*(m-1)
%o A343724     return k # _Chai Wah Wu_, May 20 2021
%Y A343724 Cf. A030098, A343725.
%K A343724 nonn,base
%O A343724 1,2
%A A343724 _Jon E. Schoenfield_, May 19 2021