A132673 a(1)=1, a(n) = 9*a(n-1) if the minimal positive integer number not yet in the sequence is greater than a(n-1), else a(n) = a(n-1) - 1.

1, 9, 8, 7, 6, 5, 4, 3, 2, 18, 17, 16, 15, 14, 13, 12, 11, 10, 90, 89, 88, 87, 86, 85, 84, 83, 82, 81, 80, 79, 78, 77, 76, 75, 74, 73, 72, 71, 70, 69, 68, 67, 66, 65, 64, 63, 62, 61, 60, 59, 58, 57, 56, 55, 54, 53, 52, 51, 50, 49, 48, 47, 46, 45, 44, 43, 42, 41, 40, 39, 38, 37

Offset

1,2

Comments

Also: a(1)=1, a(n) = maximal positive number < a(n-1) not yet in the sequence, if it exists, else a(n) = 9*a(n-1).

Also: a(1)=1, a(n) = a(n-1) - 1, if a(n-1) - 1 > 0 and has not been encountered so far, else a(n) = 9*a(n-1).

A permutation of the positive integers. The sequence is self-inverse, in that a(a(n)) = n.

Links

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

Formula

G.f.: g(x) = (x(1-2x)/(1-x) + 9x^2*f'(x^(17/8)) + (17/81)*(f'(x^(1/8)) - 9x - 1)/(1-x) where f(x) = Sum_{k>=0} x^(9^k) and f'(z) = derivative of f(x) at x = z.

a(n) = (26*9^(r/2) - 10)/8 - n if both r and s are even, else a(n) = (107*9^((s-1)/2) - 10)/8 - n, where r = ceiling(2*log_9((8n+9)/17)) and s = ceiling(2*log_9(8n+9)/8)) - 1.

a(n) = (9^floor(1 + (k+1)/2) + 17*9^floor(k/2) - 10)/8 - n, where k=r if r is odd, else k=s (with respect to r and s above; formally, k = ((r+s) - (r-s)*(-1)^r)/2).

Crossrefs

For formulas concerning a general parameter p (with respect to the recurrence rule ... a(n) = p*a(n-1) ...) see A132374.

For p=2 to p=10 see A132666 through A132674.

Sequence in context: A055120 A342115 A090671 * A363249 A107927 A019890

Adjacent sequences: A132670 A132671 A132672 * A132674 A132675 A132676

Keyword

nonn

Author

Hieronymus Fischer, Sep 15 2007

Status

approved