A113790 In each block of 5 consecutive natural numbers, swap first and 2nd and swap 4th and 5th.

2, 1, 3, 5, 4, 7, 6, 8, 10, 9, 12, 11, 13, 15, 14, 17, 16, 18, 20, 19, 22, 21, 23, 25, 24, 27, 26, 28, 30, 29, 32, 31, 33, 35, 34, 37, 36, 38, 40, 39, 42, 41, 43, 45, 44, 47, 46, 48, 50, 49, 52, 51, 53, 55, 54, 57, 56, 58, 60, 59, 62, 61, 63, 65, 64, 67, 66, 68, 70, 69, 72, 71

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

Table of n, a(n) for n=1..72.

Milan Janjic, Enumerative Formulas for Some Functions on Finite Sets

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

Sequence in context: A256371 A064429 A234751 * A181094 A282666 A181560

Adjacent sequences: A113787 A113788 A113789 * A113791 A113792 A113793

Keyword

nonn

Author

Zak Seidov, Jan 21 2006

Status

approved