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 A241916 a(2^k) = 2^k, and for other numbers, if n = 2^e1 * 3^e2 * 5^e3 * ... p_k^e_k, then a(n) = 2^(e_k - 1) * 3^(e_{k-1}) * ... * p_{k-1}^e2 * p_k^(e1+1). Here p_k is the greatest prime factor of n (A006530), and e_k is its exponent (A071178), and the exponents e1, ..., e_{k-1} >= 0. 14
 1, 2, 3, 4, 5, 9, 7, 8, 6, 25, 11, 27, 13, 49, 15, 16, 17, 18, 19, 125, 35, 121, 23, 81, 10, 169, 12, 343, 29, 75, 31, 32, 77, 289, 21, 54, 37, 361, 143, 625, 41, 245, 43, 1331, 45, 529, 47, 243, 14, 50, 221, 2197, 53, 36, 55, 2401, 323, 841, 59, 375, 61, 961, 175, 64 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS For other numbers than the powers of 2 (that are fixed), this permutation reverses the sequence of exponents in the prime factorization of n from the exponent of 2 to that of the largest prime factor, except that the exponents of 2 and the greatest prime factor present are adjusted by one. Note that some of the exponents might be zeros. Self-inverse permutation of natural numbers, composition of A122111 & A241909 in either order: a(n) = A122111(A241909(n)) = A241909(A122111(n)). This permutation preserves both bigomega and the (index of) largest prime factor: for all n it holds that A001222(a(n)) = A001222(n) and A006530(a(n)) = A006530(n) [equally: A061395(a(n)) = A061395(n)]. From the above it follows, that this fixes both primes (A000040) and powers of two (A000079), among other numbers. Even positions from n=4 onward contain only terms of A070003, and the odd positions only the terms of A102750, apart from 1 which is at a(1), and 2 which is at a(2). LINKS Antti Karttunen, Table of n, a(n) for n = 1..8192 A. Karttunen, A few notes on A122111, A241909 & A241916. FORMULA a(1)=1, and for n>1, a(n) = A006530(n) * A137502(n)/2. a(n) = A122111(A241909(n)) = A241909(A122111(n)). MATHEMATICA nn = 65; f[n_] := If[n == 1, {0}, Function[f, ReplacePart[Table[0, {PrimePi[f[[-1, 1]]]}], #] &@ Map[PrimePi@ First@ # -> Last@ # &, f]]@ FactorInteger@ n]; g[w_List] := Times @@ Flatten@ MapIndexed[Prime[#2]^#1 &, w]; Table[If[IntegerQ@ #, n/4, g@ Reverse@(# - Join[{1}, ConstantArray[0, Length@ # - 2], {1}] &@ f@ n)] &@ Log2@ n, {n, 4, 4 nn, 4}] (* Michael De Vlieger, Aug 27 2016 *) PROG (PARI) A209229(n) = (n && !bitand(n, n-1)); A241916(n) = if(1==A209229(n), n, my(f = factor(2*n), nbf = #f~, igp = primepi(f[nbf, 1]), g = f); for(i=1, nbf, g[i, 1] = prime(1+igp-primepi(f[i, 1]))); factorback(g)/2); \\ Antti Karttunen, Jul 02 2018 (Scheme) (define (A241916 n) (A122111 (A241909 n))) CROSSREFS A241912 gives the fixed points; A241913 their complement. Cf. A006530, A137502, A070003, A102750, A278525. {A000027, A122111, A241909, A241916} form a 4-group. Sequence in context: A029636 A298268 A222251 * A079871 A273291 A130064 Adjacent sequences:  A241913 A241914 A241915 * A241917 A241918 A241919 KEYWORD nonn AUTHOR Antti Karttunen, May 03 2014 EXTENSIONS Description clarified by Antti Karttunen, Jul 02 2018 STATUS approved

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Last modified October 14 04:29 EDT 2019. Contains 327995 sequences. (Running on oeis4.)