A122092 a(n) = (n-2)*a(n-1) - (n-1)*a(n-2), with a(0)=1, a(1)=1.

1, 1, -1, -3, -3, 3, 27, 117, 513, 2655, 16623, 123057, 1047717, 10048203, 106958115, 1249780653, 15892557417, 218391870807, 3224096456823, 50878586091465, 854556716966733, 15219005900538627, 286434426954471147, 5680304836232044293, 118378714577152138065

Offset

0,4

Links

G. C. Greubel, Table of n, a(n) for n = 0..350

Maple

a:= proc(n) option remember;
if n<2 then 1
elif n = 2 then -1
else (n-2)*a(n-1)-(n-1)*a(n-2)
end if; end proc;
seq(a(n), n = 0 .. 25); # G. C. Greubel, Oct 02 2019

Mathematica

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}]
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 *)

Prog

(PARI) a(n) = if(n==0 || n==1, 1, (n-2)*a(n-1) - (n-1)*a(n-2));
vector(26, n, a(n-1)) \\ G. C. Greubel, Oct 02 2019
(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
(Sage)
def a(n):
    if (n<2): return 1
    elif (n==2): return -1
    else: return (n-2)*a(n-1) - (n-1)*a(n-2)
[a(n) for n in (0..25)] # G. C. Greubel, Oct 02 2019
(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

Crossrefs

Sequence in context: A131445 A230176 A033874 * A230495 A372019 A369081

Adjacent sequences: A122089 A122090 A122091 * A122093 A122094 A122095

Keyword

sign

Author

Zak Seidov, Oct 17 2006

Extensions

Terms a(21) onward added by G. C. Greubel, Oct 02 2019

Status

approved