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%I #9 Apr 02 2023 16:29:11
%S 1,1,-1,-3,-3,3,27,117,513,2655,16623,123057,1047717,10048203,
%T 106958115,1249780653,15892557417,218391870807,3224096456823,
%U 50878586091465,854556716966733,15219005900538627,286434426954471147,5680304836232044293,118378714577152138065
%N a(n) = (n-2)*a(n-1) - (n-1)*a(n-2), with a(0)=1, a(1)=1.
%H G. C. Greubel, <a href="/A122092/b122092.txt">Table of n, a(n) for n = 0..350</a>
%p a:= proc(n) option remember;
%p if n<2 then 1
%p elif n = 2 then -1
%p else (n-2)*a(n-1)-(n-1)*a(n-2)
%p end if; end proc;
%p seq(a(n), n = 0 .. 25); # _G. C. Greubel_, Oct 02 2019
%t a[0]=1;a[1]=1;a[n_]:=a[n]=(n-2)a[n-1]-(n-1)a[n-2]; Table[a[n],{n,0,20}]
%t nxt[{n_,a_,b_}]:={(n+1),b,b(n-1)-a n}; NestList[nxt,{1,1,1},30][[;;,2]] (* _Harvey P. Dale_, Apr 02 2023 *)
%o (PARI) a(n) = if(n==0 || n==1, 1, (n-2)*a(n-1) - (n-1)*a(n-2));
%o vector(26, n, a(n-1)) \\ _G. C. Greubel_, Oct 02 2019
%o (Magma) I:=[1,1,-1]; [n le 3 select I[n] else (n-3)*Self(n-1) - (n-2)*Self(n-2): n in [1..30]]; // _G. C. Greubel_, Oct 02 2019
%o (Sage)
%o def a(n):
%o if (n<2): return 1
%o elif (n==2): return -1
%o else: return (n-2)*a(n-1) - (n-1)*a(n-2)
%o [a(n) for n in (0..25)] # _G. C. Greubel_, Oct 02 2019
%o (GAP) a:=[1,1,-1];; for n in [4..25] do a[n]:=(n-3)*a[n-1]-(n-2)*a[n-2]; od; a; # _G. C. Greubel_, Oct 02 2019
%K sign
%O 0,4
%A _Zak Seidov_, Oct 17 2006
%E Terms a(21) onward added by _G. C. Greubel_, Oct 02 2019
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