A239967 a(n) = a(n-1)*a(n-2) - a(n-3)*a(n-4), starting with 1,1,2,2.
1, 1, 2, 2, 3, 4, 8, 26, 196, 5064, 992336, 5025184408, 4986671393704544, 25058943335463698842360448, 124960715887419957580506993886784668778624, 3131383498581835095655557937198087383978439131276209799741457913600
Offset: 1
Keywords
Crossrefs
Cf. A111235.
Programs
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Magma
I:=[1,1,2,2]; [n le 4 select I[n] else Self(n-1)*Self(n-2)-Self(n-3)*Self(n-4): n in [1..16]]; // Vincenzo Librandi, Mar 30 2014
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PARI
al(n)=local(v=vector(n));v[1]=v[2]=1;v[3]=v[4]=2;for(i=5,n,v[i]=v[i-1]*v[i-2]-v[i-3]*v[i-4]);v