A048347 a(n)^2 is the smallest square containing exactly n 2's.

5, 15, 149, 1415, 4585, 14585, 105935, 364585, 3496101, 4714045, 34964585, 149305935, 1490725415, 4714469665, 1490711985, 149071333335, 1105537083332, 1489973900149, 15106363633335, 47140462469223

Offset

1,1

Comments

a(19) > 10^13. - Jon E. Schoenfield, Dec 25 2008

a(21) > 10^14. - Giovanni Resta, Jul 27 2018

Links

Table of n, a(n) for n=1..20.

Eric Weisstein's World of Mathematics, Square Number

Example

From Jon E. Schoenfield, Dec 25 2008: (Start)
a(16) = 149071333335 = sqrt(22222262422274682222225);
a(17) = 1105537083332 = sqrt(1222212242622225512222224);
a(18) = 1489973900149 = sqrt(2220022223125222222222201). (End)
From Giovanni Resta, Jul 27 2018: (Start)
a(19) = 15106363633335 = sqrt(228202222222546222323222225);
a(20) = 47140462469223 = sqrt(2222223201812222222222223729). (End)

Mathematica

a[n_] := Block[{k=1}, While[DigitCount[k^2, 10, 2] != n, k++]; k]; Array[a, 7] (* Giovanni Resta, Jul 27 2018 *)

Crossrefs

Cf. A036509, A034980.

Sequence in context: A207970 A207971 A245648 * A034980 A174850 A143048

Adjacent sequences: A048344 A048345 A048346 * A048348 A048349 A048350

Keyword

nonn,base,more

Author

Patrick De Geest, Mar 15 1999

Extensions

a(13)-a(15) from Max Alekseyev, Oct 20 2008, Nov 10 2008, Dec 05 2008

a(16)-a(18) from Jon E. Schoenfield, Dec 25 2008

a(19)-a(20) from Giovanni Resta, Jul 27 2018

Status

approved