OFFSET
1,1
COMMENTS
Permutation of natural numbers. Or five arithmetic progressions interlaced with b(1)=2,1,3,5,4 and d=b(n+1)-b(n)=5
For n>=1, a(n) is equal to the number of functions f:{1,2,3}->{1,2,...,n+1} such that for fixed different x_1, x_2 in {1,2,3} and fixed y_1, y_2 in {1,2,...,n+1} we have f(x_1)<>y_1 and f(x_2)<>y_2. - Milan Janjic, Apr 17 2007
LINKS
FORMULA
a(n)=m+(1/2)*(n-m)(5-(n-m)^2); m=3+5*floor((n-1)/5); n=1, 2, ...
MATHEMATICA
m:=3+5*Floor[(n-1)/5]; Table[m+(1/2)*(n-m)*(5-(n-m)^2), {n, 1, 80}]
CROSSREFS
KEYWORD
nonn
AUTHOR
Zak Seidov, Jan 21 2006
STATUS
approved