A065992 - OEIS

%I #12 Jun 17 2018 01:53:30

%S 6,12,31,40,411,1638,2352,9465,263284,647498,1227466,3179974,4027111,

%T 5382959,29444851,32585267,50402546,52610042,98185445,3750320631,

%U 14471636075,79389982446,157183654414

%N Numbers k such that the sum of the nonsquare numbers from 1 to k is a square.

%C a(24) > 2*10^12. - _Donovan Johnson_, Jan 16 2011

%e 12 is in the sequence because 2 + 3 + 5 + 6 + 7 + 8 + 10 + 11 + 12 = 64 = 8*8.

%o (PARI) isA065992(n) = {local(m);m=floor(sqrt(n));if(n<2,0,issquare((n*(n+1)/2)-(m*(m+1)*(2*m+1)/6)))} \\ _Michael B. Porter_, Feb 02 2010

%K nonn

%O 1,1

%A Jonathan Ayres (jonathan.ayres(AT)ntlworld.com), Dec 10 2001

%E a(20)-a(23) from _Donovan Johnson_, Jan 16 2011