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 A257727 Permutation of natural numbers: a(1) = 1, a(oddprime(n)) = 1 + 2*a(n), a(not_an_oddprime(n)) = 2*a(n-1). 4
 1, 2, 3, 4, 5, 6, 7, 8, 10, 12, 9, 14, 11, 16, 20, 24, 13, 18, 15, 28, 22, 32, 17, 40, 48, 26, 36, 30, 21, 56, 25, 44, 64, 34, 80, 96, 19, 52, 72, 60, 29, 42, 23, 112, 50, 88, 33, 128, 68, 160, 192, 38, 41, 104, 144, 120, 58, 84, 49, 46, 27, 224, 100, 176, 66, 256, 37, 136, 320, 384, 31, 76, 57, 82, 208, 288, 240, 116, 45 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS Here oddprime(n) = n-th odd prime = A065091(n) = A000040(n+1), not_an_oddprime(n) = n-th natural number which is not an odd prime = A065090(n). LINKS Antti Karttunen, Table of n, a(n) for n = 1..10000 FORMULA a(1) = 1; a(2) = 2; and for n > 2, if A010051(n) = 1 [i.e., when n is a prime], then a(n) = 1 + 2*a(A000720(n)-1), otherwise a(n) = 2*a(A062298(n)). As a composition of other permutations: a(n) = A246377(A257730(n)). a(n) = A257725(A257801(n)). EXAMPLE For n=2, which is the second natural number >= 1 that is not an odd prime [2 = A065090(2)], we compute 2*a(1) = 2 = a(2). For n=4, which is A065090(3), we compute 2*a(3-1) = 2*2 = 4. For n=5, and 5 is the second odd prime [5 = A065091(2)], thus a(5) = 1 + 2*a(2) = 5. For n=9, which is the sixth natural number >= 1 not an odd prime (9 = A065090(6)), we compute 2*a(6-1) = 2*5 = 10. For n=11, which is the fourth odd prime [11 = A065091(4)], we compute 1 + 2*a(4) = 1 + 2*4 = 9, thus a(11) = 9. PROG (Scheme, with memoizing definec-macro) (definec (A257727 n) (cond ((<= n 2) n) ((= 1 (A010051 n)) (+ 1 (* 2 (A257727 (+ -1 (A000720 n)))))) (else (* 2 (A257727 (A062298 n)))))) CROSSREFS Inverse: A257728. Cf. A000720, A010051, A062298, A065090, A065091. Related or similar permutations: A246377, A246378, A257725, A257730, A257801. Sequence in context: A229769 A130676 A039227 * A039273 A039164 A263977 Adjacent sequences:  A257724 A257725 A257726 * A257728 A257729 A257730 KEYWORD nonn AUTHOR Antti Karttunen, May 09 2015 STATUS approved

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Last modified May 14 17:44 EDT 2021. Contains 343900 sequences. (Running on oeis4.)