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 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. 7

%I

%S 3,4,12,84,132,12324,1836,105552,255084,197580,10358340,13775220,

%T 1936434780,51299286012,123205977516,862441842612,1310543298204,

%U 667510076211780,207181940072172,110912831751840,1698410314006284

%N 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.

%H Vincenzo Librandi, <a href="/A018930/b018930.txt">Table of n, a(n) for n = 1..100</a>

%t 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 *)

%o (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

%Y Cf. A018928, A018929, A127689.

%K nonn

%O 1,1

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

%E More terms from _David W. Wilson_.

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Last modified November 28 08:48 EST 2022. Contains 358407 sequences. (Running on oeis4.)