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A113790 In each block of 5 consecutive natural numbers, swap first and 2nd and swap 4th and 5th. 0

%I

%S 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,

%T 26,28,30,29,32,31,33,35,34,37,36,38,40,39,42,41,43,45,44,47,46,48,50,

%U 49,52,51,53,55,54,57,56,58,60,59,62,61,63,65,64,67,66,68,70,69,72,71

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

%C 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

%C 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

%H Milan Janjic, <a href="http://www.pmfbl.org/janjic/">Enumerative Formulas for Some Functions on Finite Sets</a>

%F a(n)=m+(1/2)*(n-m)(5-(n-m)^2); m=3+5*floor((n-1)/5); n=1, 2, ...

%t m:=3+5*Floor[(n-1)/5]; Table[m+(1/2)*(n-m)*(5-(n-m)^2), {n, 1, 80}]

%K nonn

%O 1,1

%A _Zak Seidov_, Jan 21 2006

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Last modified December 5 09:03 EST 2020. Contains 338944 sequences. (Running on oeis4.)