A154448 - OEIS

A154448

Permutation of nonnegative integers induced by wreath recursion a=s(b,c), b=s(c,a), c=(c,c), starting from state a, rewriting bits from the second most significant bit toward the least significant end.

0, 1, 3, 2, 7, 6, 4, 5, 14, 15, 13, 12, 8, 9, 10, 11, 28, 29, 30, 31, 27, 26, 24, 25, 16, 17, 18, 19, 20, 21, 22, 23, 56, 57, 58, 59, 60, 61, 62, 63, 54, 55, 53, 52, 48, 49, 50, 51, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 112, 113, 114, 115, 116, 117

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OFFSET

0,3

COMMENTS

This permutation of natural numbers is induced by the first generator of group 2861 mentioned on page 144 of "Classification of groups generated by 3-state automata over a 2-letter alphabet" paper. It can be computed by starting scanning n's binary expansion rightward from the second most significant bit, complementing every bit down to and including A) either the first 0-bit at even distance from the most significant bit or B) the first 1-bit at odd distance from the most significant bit.

LINKS

Antti Karttunen, Table of n, a(n) for n = 0..2047

Bondarenko, Grigorchuk, Kravchenko, Muntyan, Nekrashevych, Savchuk, and Sunic, Classification of groups generated by 3-state automata over a 2-letter alphabet, arXiv:0803.3555 [math.GR], 2008, p. 144.

Index entries for sequences that are permutations of the natural numbers

EXAMPLE

25 = 11001 in binary, the first zero-bit at odd distance from the msb is immediately at where we start (at the second most significant bit), so we complement it and fix the rest, yielding 10001 (17 in binary), thus a(25)=17.

PROG

(MIT/GNU Scheme) (define (A154448 n) (if (< n 2) n (let loop ((maskbit (A072376 n)) (p 1) (z n)) (cond ((zero? maskbit) z) ((= p (modulo (floor->exact (/ n maskbit)) 2)) (+ z (* (- 1 (* 2 p)) maskbit))) (else (loop (floor->exact (/ maskbit 2)) (- 1 p) (- z (* (- 1 (* 2 p)) maskbit))))))))

(R)

maxlevel <- 5 # by choice

a <- 1

for(m in 0:maxlevel) {

for(k in 0:(2^m-1)){

a[2^(m+1) + 2*k ] <- 2*a[2^m + k]

a[2^(m+1) + 2*k + 1] <- 2*a[2^m + k] + 1

}

x <- floor(2^(m+2)/3)

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

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

}