A092401 List of pairs n, 3n, where n is the least unused number so far.

1, 3, 2, 6, 4, 12, 5, 15, 7, 21, 8, 24, 9, 27, 10, 30, 11, 33, 13, 39, 14, 42, 16, 48, 17, 51, 18, 54, 19, 57, 20, 60, 22, 66, 23, 69, 25, 75, 26, 78, 28, 84, 29, 87, 31, 93, 32, 96, 34, 102, 35, 105, 36, 108, 37, 111, 38, 114, 40, 120, 41, 123, 43, 129, 44, 132, 45, 135, 46

Offset

1,2

Comments

A permutation of the natural numbers.

Links

T. D. Noe, Table of n, a(n) for n=1..1000

Index entries for sequences that are permutations of the natural numbers

Formula

a(2n-1) = A007417(n), a(2n) = 3*A007417(n).

Mathematica

A007417 = Select[ Range[100], (# // IntegerDigits[#, 3]& // Split // Last // Count[#, 0]& // EvenQ)&]; a[n_] := If[ OddQ[n], A007417[[(n+1)/2]], 3*A007417[[n/2]] ]; Table[a[n], {n, 1, 100}] (* Jean-François Alcover, Mar 01 2013, from formula *)

Prog

(Haskell)
import Data.List (delete)
a092401 n = a092401_list !! (n-1)
a092401_list = f [1..] where
   f (x:xs) = x : x' : f (delete x' xs) where x' = 3*x
-- Reinhard Zumkeller, Jan 03 2012
(Python)
from sympy import integer_log
def A092401(n):
    def f(x): return (n+1>>1)+x-sum(((m:=x//9**i)-2)//3+(m-1)//3+2 for i in range(integer_log(x, 9)[0]+1))
    m, k = n+1>>1, f(n+1>>1)
    while m != k: m, k = k, f(k)
    return m if n&1 else 3*m # Chai Wah Wu, Feb 16 2025

Crossrefs

Cf. A036552, A203602 (inverse).

Sequence in context: A258212 A347704 A355019 * A222208 A302853 A116626

Adjacent sequences: A092398 A092399 A092400 * A092402 A092403 A092404

Keyword

easy,nice,nonn

Author

Philippe Deléham, Mar 22 2004

Status

approved