A036507 Smallest square containing exactly n decimal digits '0'.

0, 100, 102400, 10000, 10240000, 1000000, 1024000000, 100000000, 102400000000, 10000000000, 10240000000000, 1000000000000, 1024000000000000, 100000000000000, 102400000000000000, 10000000000000000, 10240000000000000000, 1000000000000000000, 1024000000000000000000

Offset: 1

Author

David W. Wilson

Keywords

• nonn
• base
• easy
• changed

Crossrefs

Cf. A036508 (digits 1), A036509 (2), A036510 (3), A036511 (4), A036512 (5), A036513 (6), A036514 (7), A036515 (8), A036516 (9).

Programs

Mathematica

nsmall = Table[Infinity, 20];
For[i = 0, i <= 4*10^6, i++, n0 = Count[IntegerDigits[i^2], 0];
  If[nsmall[[n0]] > i^2, nsmall[[n0]] = i^2]];
ReplaceAll[nsmall, Infinity -> "?"] (* Robert Price, Mar 22 2020 *)
a[n_] := If[OddQ[n], 1024*10^(n-1), 10^n]; a[1] = 0; Array[a, 20] (* Amiram Eldar, Aug 26 2025 *)

Formula

a(2*n) = 10^(2*n), a(2*n+1) = 1024*10^(2*n) for n >= 1 since 1024 is the smallest square factor that contains a single '0'. - Georg Fischer, Jul 03 2023

Sum_{n>=2} 1/a(n) = 1025/101376. - Amiram Eldar, Aug 26 2025