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 A135141 a(1)=1, a(p_n)=2*a(n), a(c_n)=2*a(n)+1, where p_n = n-th prime, c_n = n-th composite number. 35
 1, 2, 4, 3, 8, 5, 6, 9, 7, 17, 16, 11, 10, 13, 19, 15, 12, 35, 18, 33, 23, 21, 14, 27, 39, 31, 25, 71, 34, 37, 32, 67, 47, 43, 29, 55, 22, 79, 63, 51, 20, 143, 26, 69, 75, 65, 38, 135, 95, 87, 59, 111, 30, 45, 159, 127, 103, 41, 24, 287, 70, 53, 139, 151, 131, 77, 36, 271, 191 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS A permutation of the positive integers, related to A078442. a(p) is even when p is prime and is divisible by 2^(prime order of p). What is the length of the cycle containing 10? Is it infinite? The cycle begins 10, 17, 12, 11, 16, 15, 19, 18, 35, 29, 34, 43, 26, 31, 32, 67, 36, 55, 159, 1055, 441, 563, 100, 447, 7935, 274726911, 1013992070762272391167, ... Implementation in Mmca: NestList[a(AT)# &, 10, 26] Furthermore, it appears that any non-single-digit number has an infinite cycle. (* Robert G. Wilson v, Feb 16 2008 *) Records: 1, 2, 4, 8, 9, 17, 19, 35, 39, 71, 79, 143, 159, 287, 319, 575, 639, 1151, 1279, 2303, 2559, 4607, 5119, 9215, 10239, 18431, 20479, 36863, 40959, 73727, 81919, 147455, 163839, 294911, 327679, 589823, 655359, ..., . (* Robert G. Wilson v, Feb 16 2008 *) LINKS Robert G. Wilson v, Table of n, a(n) for n = 1..10000 FORMULA a(n) = 2*A135141((A049084(n))*chip + A066246(n)*(1-chip)) + 1 - chip, where chip = a010051(n). - Reinhard Zumkeller, Jan 29 2014 EXAMPLE a(20)=33=2*16+1 because 20 is 11th composite and a(11)=16. Or, a(20)=33=100001(bin). In other words it is a composite number, its index is a prime number, whose index is a prime.... MATHEMATICA a = 1; a[n_] := If[PrimeQ@n, 2*a[PrimePi[n]], 2*a[n - 1 - PrimePi@n] + 1]; Array[a, 69] (* Robert G. Wilson v, Feb 16 2008 *) PROG (Maxima) Let pc = prime count (which prime it is), cc = composite count: pc:0; cc:0; pc:1; cc:1; pc[n]:=if primep(n) then 1+pc[prev_prime(n)] else 0; cc[n]:=if primep(n) then 0 else if primep(n-1) then 1+cc[n-2] else 1+cc[n-1]; a:1; a[n]:=if primep(n) then 2*a[pc[n]] else 1+2*a[cc[n]]; (Haskell) import Data.List (genericIndex) a135141 n = genericIndex a135141_list (n-1) a135141_list = 1 : map f [2..] where    f x | iprime == 0 = 2 * (a135141 \$ a066246 x) + 1        | otherwise   = 2 * (a135141 iprime)        where iprime = a049084 x -- Reinhard Zumkeller, Jan 29 2014 (Python) from sympy import isprime, primepi def a(n): return 1 if n==1 else 2*a(primepi(n)) if isprime(n) else 2*a(n - 1 - primepi(n)) + 1 # Indranil Ghosh, Jun 11 2017, after Mathematica code CROSSREFS Cf. A078442. Cf. A227413 (inverse), A026238. Sequence in context: A207790 A305427 A269375 * A098709 A054238 A225589 Adjacent sequences:  A135138 A135139 A135140 * A135142 A135143 A135144 KEYWORD nonn,look AUTHOR Katarzyna Matylla, Feb 13 2008 STATUS approved

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Last modified October 14 05:08 EDT 2019. Contains 327995 sequences. (Running on oeis4.)