A018930 a(1)=3; for n>1, a(n) is smallest positive integer such that a(1)^2+...+a(n)^2 = m^2 for some m.

3, 4, 12, 84, 132, 12324, 1836, 105552, 255084, 197580, 10358340, 13775220, 1936434780, 51299286012, 123205977516, 862441842612, 1310543298204, 667510076211780, 207181940072172, 110912831751840, 1698410314006284

Offset: 1

Author

Charles Reed (charles.reed(AT)bbs.ewgateway.org)

Keywords

• nonn

Links

• Vincenzo Librandi, Table of n, a(n) for n = 1..100

Crossrefs

Cf. A018928, A018929, A127689.

Programs

Mathematica

f[n_]:=Module[{a={3}}, Do[AppendTo[a,First[y/. {ToRules[Reduce[{y^2+a.a == x^2,x>0,y>0}, {y,x},Integers]]}]], {n-1}]; a]; f[21]//Timing (* Jean-François Alcover, Jan 26 2007 *)

PARI

print1("3, "); s=9; for(n=1,30, d=divisors(s); t=d[#d\2]; q=(s\t-t)/2; print1(q,", "); s+=q^2); \\ Max Alekseyev, Nov 23 2012

Extensions

More terms from David W. Wilson.