OFFSET
1,2
COMMENTS
a(a(n))=n (a self-inverse permutation);
for n>1: a(n) = n iff n == 0 modulo 5.
LINKS
Eric Weisstein's World of Mathematics, Alternating Permutations
Reinhard Zumkeller, Illustration for A074066-A074068
Index entries for linear recurrences with constant coefficients, signature (1,0,0,0,1,-1)
FORMULA
a(n) = 5*floor(n/5) + 10*floor((n mod 5)/3) - (n mod 5) for n>2; a(n)=n for n<=2.
a(n) = a(n-1) + a(n-5) - a(n-6) for n > 8. - Chai Wah Wu, May 25 2016
g.f.: x+2*x + x^3*(7-x-x^2-x^3-x^4+2*x^5) / ( (x^4+x^3+x^2+x+1)*(x-1)^2 ). - R. J. Mathar, May 22 2019
MATHEMATICA
{1, 2}~Join~Flatten[Reverse /@ Partition[Range[3, 72], 5]] (* after Harvey P. Dale at A074066, or *)
{1, 2}~Join~Table[5 Floor[n/5] + 10 Floor[#/3] - # &@ Mod[n, 5], {n, 3, 69}] (* Michael De Vlieger, May 25 2016 *)
PROG
(Haskell)
a074067 n = a074067_list !! (n-1)
a074067_list = 1 : 2 : xs where xs = 7 : 6 : 5 : 4 : 3 : map (+ 5) xs
-- Reinhard Zumkeller, Feb 21 2011
CROSSREFS
KEYWORD
nonn,easy
AUTHOR
Reinhard Zumkeller, Aug 17 2002
STATUS
approved