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 A125974 Function whose restriction to A014486 induces signature-permutation A125976. 3
 0, 1, 2, 5, 10, 3, 2, 21, 42, 11, 12, 13, 10, 11, 10, 85, 170, 43, 44, 13, 52, 7, 6, 53, 42, 11, 12, 45, 10, 43, 42, 341, 682, 171, 172, 45, 180, 39, 38, 53, 212, 23, 56, 57, 50, 51, 22, 213, 170, 43, 44, 45, 52, 39, 38, 181, 42, 43, 44, 173, 42, 171, 170, 1365, 2730 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,3 COMMENTS A125975 gives the terms i, for which a(a(i)) = i. Question: would it be possible to construct a more elegant and natural variant which were an involution for all the natural numbers? (and acting in the same way on the set A125975, or at least on the set A014486.) LINKS PROG (Scheme:) (define (A125974 n) (let ((runlens (binexp->runcount1list n))) (let loop ((chosen (reverse! (bisect runlens 0))) (others (reverse! (bisect runlens 1))) (s 0) (b (modulo n 2)) (p 1)) (cond ((and (null? chosen) (null? others)) s) ((and (pair? chosen) (= 1 (car chosen)) (pair? (cdr chosen))) (loop (cdr chosen) others (+ s (* b p)) b (+ p p))) (else (loop others (if (or (null? chosen) (= 1 (car chosen))) '() (cons (- (car chosen) 1) (cdr chosen))) (+ s (* b p)) (- 1 b) (+ p p))))))) (Scheme:) (define (binexp->runcount1list n) (if (zero? n) (list) (let loop ((n n) (rc (list)) (count 0) (prev-bit (modulo n 2))) (if (zero? n) (cons count rc) (if (eq? (modulo n 2) prev-bit) (loop (floor->exact (/ n 2)) rc (+ 1 count) (modulo n 2)) (loop (floor->exact (/ n 2)) (cons count rc) 1 (modulo n 2))))))) ;; (binexp->runcount1list 25) returns (2 2 1) (Scheme:) (define (bisect lista parity) (let loop ((lista lista) (i 0) (z (list))) (cond ((null? lista) (reverse! z)) ((eq? i parity) (loop (cdr lista) (modulo (1+ i) 2) (cons (car lista) z))) (else (loop (cdr lista) (modulo (1+ i) 2) z))))) (Python, replace leading dots by blanks before running:) .def A125974(n): .. if 0 == n: return(n) .. chosen = A000265(n) # Initially ones, get rid of lsb-0's. .. others = n >> A007814(n+1) # Initially zeros, get rid of lsb-1's. .. s = 0 # the resulting sum .. b = n%2 # n's parity. .. p = 1 # powers of two. .. while (chosen != 0) or (others != 0): .... if (1 == chosen) or (1 == A036987(chosen+1)): # Last one or zero at hand. ....... chosen = others ....... others = 0 ....... nb = 1 - b .... elif (0 == (chosen%4)) or (3 == (chosen%4)): # Source run continues, dest changes. ....... tmp = chosen ....... chosen = others ....... others = tmp >> 1 ....... nb = 1 - b .... elif (1 == (chosen%4)): # Source run changes, from ones to zeros, skip past zeros. ....... chosen = A000265(chosen-1) ....... nb = b .... else: # Source run changes, from zeros to ones, skip past ones. ....... chosen = chosen >> A007814(chosen+2) ....... nb = b .... s += b*p .... p <<= 1 .... b = nb .. return(s) CROSSREFS Python code uses the following functions: A000265, A007814 and A036987. Sequence in context: A188948 A083460 A172411 * A218336 A059955 A099796 Adjacent sequences:  A125971 A125972 A125973 * A125975 A125976 A125977 KEYWORD nonn,base AUTHOR Antti Karttunen, Jan 02 2007 STATUS approved

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