A258746 - OEIS

A258746

Permutation of the positive integers: this permutation transforms the enumeration system of positive irreducible fractions A007305/A047679 (Stern-Brocot) into the enumeration system A162909/A162910 (Bird), and vice versa.

1, 2, 3, 5, 4, 7, 6, 10, 11, 8, 9, 14, 15, 12, 13, 21, 20, 23, 22, 17, 16, 19, 18, 29, 28, 31, 30, 25, 24, 27, 26, 42, 43, 40, 41, 46, 47, 44, 45, 34, 35, 32, 33, 38, 39, 36, 37, 58, 59, 56, 57, 62

(list; graph; refs; listen; history; text; internal format)

OFFSET

1,2

COMMENTS

As A117120 the permutation is self-inverse. Except for fixed points 1, 2, 3 it consists completely of 2-cycles: (4,5), (6,7), (8,10), (9,11), (12,14), (13,15), (16,21), (17,20), ..., (24,29), ..., (32,42), ... .

LINKS

Yosu Yurramendi, Table of n, a(n) for n = 1..20001

Index entries for sequences that are permutations of the natural numbers

FORMULA

a(1) = 1, a(2) = 2, a(3) = 3. For n >= 2, m = floor(log_2(n)). If m even, then a(2*n) = 2*a(n) and a(2*n+1) = 2*a(n)+1. If m odd, then a(2*n) = 2*a(n)+1 and a(2*n+1) = 2*a(n).

From Yosu Yurramendi, Mar 23 2017: (Start)

A258996(a(n)) = a(A258996(n)) for n > 0;

A117120(a(n)) = a(A117120(n)) for n > 0;

A092569(a(n)) = a(A092569(n)) for n > 0;

A063946(a(n)) = a(A063946(n)) for n > 0;

A054429(a(n)) = a(A054429(n)) = A165199(n) for n > 0;

A065190(a(n)) = a(A065190(n)) for n > 0. (End)

PROG

(R)

a <- 1:3

maxn <- 50 # by choice

for(n in 2:maxn){

m <- floor(log2(n))

if(m%%2 == 0) {

a[2*n ] <- 2*a[n]

a[2*n+1] <- 2*a[n]+1 }

else {

a[2*n ] <- 2*a[n]+1

a[2*n+1] <- 2*a[n] }

}

# Yosu Yurramendi, Jun 09 2015

(R)

# Given n, compute a(n) by taking into account the binary representation of n

maxblock <- 7 # by choice

a <- 1:3

for(n in 4:2^maxblock){

ones <- which(as.integer(intToBits(n)) == 1)

nbit <- as.integer(intToBits(n))[1:tail(ones, n = 1)]

anbit <- nbit

ifelse(floor(log2(n)) %% 2 == 0,

anbit[seq(1, length(anbit)-1, 2)] <- 1 - anbit[seq(1, length(anbit)-1, 2)],

anbit[seq(2, length(anbit) - 1, 2)] <- 1 - anbit[seq(2, length(anbit)-1, 2)])

a <- c(a, sum(anbit*2^(0:(length(anbit)-1))))

}

# Yosu Yurramendi, May 29 2021

CROSSREFS

Cf. A117120.

Sequence in context: A261102 A101212 A232641 * A064706 A100282 A100281

Adjacent sequences: A258743 A258744 A258745 * A258747 A258748 A258749

KEYWORD

nonn

AUTHOR

Yosu Yurramendi, Jun 09 2015

STATUS

approved