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%I #12 Sep 26 2013 09:01:53
%S 1,2,3,4,5,67,150,154,387,547,813,1034,1710,4994,13582,700427,1598953,
%T 2960411
%N Let u(1) = u(2) = u(3) = u(4) = 1, u(n+4)*(n+4) = u(n+3)*(n+3)+u(n+2)*(n+2)+u(n+1)*(n+1)+u(n)*n; sequence gives values of n such that u(n) is an integer.
%C No more terms up to n=15*10^6. - Lambert Klasen (Lambert.Klasen(AT)gmx.net) and _Robert G. Wilson v_, Aug 05 2005
%t a = {0, 1, 1, 1, 1}; Do[a = Rest[ Join[a, {((n - 4)a[[2]] + (n - 3)a[[3]] + (n - 2)a[[4]] + (n - 1)a[[5]])/n}]]; If[ IntegerQ[ Last[ a]], Print[n]], {n, 5, 2*10^6}] (* _Robert G. Wilson v_ *)
%o (PARI) v = [1,1,1,1];for(k=0,15,s=k*10^6+1;e=(k+1)*10^6;if(s==1,s=5);print(s," - ",e,":");for(n=s,e,v[(n-1)%4+1]=((n-4)*v[(n-1)%4+1]+(n-3)*v[(n)%4+1]+(n-2)*v[(n+1)%4+1]+(n-1)*v[(n+2)%4+1])/n;if(denominator(v[(n-1)%4+1])==1,print1(n,",")));print()) \\ (Klasen)
%Y Cf. A075770.
%K nonn
%O 1,2
%A _Benoit Cloitre_, Oct 14 2002
%E Corrected and extended by Lambert Klasen (Lambert.Klasen(AT)gmx.net) and _Robert G. Wilson v_, Aug 05 2005
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