OFFSET
0,3
EXAMPLE
For a(4), we have 2+3=1+4, so a(4)=4! - (23)(14) - (14)(32) = 24 - 4 - 4 = 16.
For a(5), we have 1+4=2+3, 1+5=2+4, 2+5=3+4 to avoid.
PROG
(PARI) { for (i=3, 9, v=vector(i-1); c=0; for (j=1, i!, x=numtoperm(i, j); for (k=1, i-1, v[k]=x[k]+x[k+1]); fl=0; v=vecsort(v); for (z=1, i-2, if (v[z]==v[z+1], fl=1; break)); if (fl==0, c++)); print1(", "c)) }
CROSSREFS
KEYWORD
hard,more,nonn
AUTHOR
Jon Perry, Feb 23 2004
EXTENSIONS
a(10)-a(11) from John W. Layman, Mar 10 2004
More terms from R. H. Hardin, Nov 11 2008
Definition corrected by Emeric Deutsch, Nov 28 2008
a(0)=1 prepended by Alois P. Heinz, Jul 06 2021
STATUS
approved