1,2

For n>0 a(n) = number of permutations of length n+1 that have 2 predetermined elements non-adjacent, e.g. for n=2, the permutations with say 1 and 2 non-adjacent are 132 and 231, therefore a(2)=2. - Jon Perry (perry(AT)globalnet.co.uk), Jun 08 2003

Number of multiplications performed in a determinant. [From Mats O. Granvik (mgranvik(AT)abo.fi), Sep 12 2008]

Harry J. Smith, Table of n, a(n) for n=1,...,100

Milan Janjic, Enumerative Formulas for Some Functions on Finite Sets

S. R. Schmitt Determinants [From Mats O. Granvik (mgranvik(AT)abo.fi), Sep 12 2008]

a:=n->sum(n!, j=2..n):seq(a(n), n=1..19); - Zerinvary Lajos (zerinvarylajos(AT)yahoo.com), Apr 30 2007

seq(sum(mul(j, j=1..n), k=2..n), n=1..19); - Zerinvary Lajos (zerinvarylajos(AT)yahoo.com), Jun 01 2007

a:=n->add((n!), j=1..n-1):seq(a(n), n=1..21); [From Zerinvary Lajos (zerinvarylajos(AT)yahoo.com), Aug 27 2008]

restart: G(x):=x^2/(1-x)^2: f[0]:=G(x): for n from 1 to 19 do f[n]:=diff(f[n-1], x) od: x:=0: seq(f[n], n=1..19); # [From Zerinvary Lajos (zerinvarylajos(AT)yahoo.com), Apr 01 2009]

a[n_]:=1*(n+2)!-2*(n+1)!; ..and/or.. a[n_]:=n!*(n-1); [From Vladimir Orlovsky (4vladimir(AT)gmail.com), Dec 05 2008]

Table[Sum[n!, {i, 2, n}], {n, 1, 19}] [From Zerinvary Lajos (zerinvarylajos(AT)yahoo.com), Jul 12 2009]

(PARI) { f=1; for (n=1, 100, f*=n; write("b062119.txt", n, " ", f*(n - 1)) ) } [From Harry J. Smith (hjsmithh(AT)sbcglobal.net), Aug 02 2009]

Cf. A018931.

a(n)=2*A001286(n). Cf. A052849.

A001563 [From Zerinvary Lajos (zerinvarylajos(AT)yahoo.com), Aug 27 2008]

Sequence in context: A119921 A167747 A018931 this_sequence A052556 A052833 A144086

Adjacent sequences: A062116 A062117 A062118 this_sequence A062120 A062121 A062122

easy,nonn

Olivier Gerard (olivier.gerard(AT)gmail.com), Jun 13 2001

Last term a(19) corrected by Harry J. Smith (hjsmithh(AT)sbcglobal.net), Aug 02 2009