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 A127668 Concatenated indices of primes in prime factorization of n. 4
 1, 2, 11, 3, 21, 4, 111, 22, 31, 5, 211, 6, 41, 32, 1111, 7, 221, 8, 311, 42, 51, 9, 2111, 33, 61, 222, 411, 10, 321, 11, 11111, 52, 71, 43, 2211, 12, 81, 62, 3111, 13, 421, 14, 511, 322, 91, 15, 21111, 44, 331, 72, 611, 16, 2221, 53, 4111, 82, 101, 17, 3211, 18, 111 (list; graph; refs; listen; history; text; internal format)
 OFFSET 2,2 COMMENTS For each n>=2 the indices i of primes p(i), i>=1, in the prime number decomposition of n are ordered from right to left. The mapping n->a(n) is from {2,3,...} onto {1,2,3,...}=N but not injective; hence not invertible. There are at most pa(k):=A000041(k) (partition numbers) different numbers which map to any a(n) with k digits. 10 and 12 are the smallest numbers for which this is not equality; 10 because 1,0 is not a partition, and 12 because 1,2 lists partition parts in the wrong order. For the invertible map onto lists of prime number indices see the W. Lang link; also A112798. LINKS W. Lang: Unique representation as lists. FORMULA If n=p_1^(n_1) p_2^(n_2)...p_k^(n_k), with n_j>=0, then a(n) = n_k times k followed by n_{k-1} times (k-1)... followed by n_1 times 1. EXAMPLE 111=a(2*2*2)=a(31*2)=a(607). 111 has k=3 digits, hence pa(3)=3 different numbers are mapped to it. a(5)=3 because 5=p(3). a(4)=11 because 4=2*2=p(1)*p(1). Also a(31)=11 because p(11)=31. CROSSREFS For numbers with no prime divisor > 23, the sum of digits gives A056239(n), n>=2. For numbers with no prime divisor > 23, the length of the digits gives A001222(n), n>=2, (number of prime divisors of n). The number of numbers mapped to a(n) gives A127669. Cf. A054841(n), n>=2: exponents in prime decomposition of n. See A112798 for another version of this data. Sequence in context: A113625 A279237 A090323 * A261300 A068743 A322761 Adjacent sequences:  A127665 A127666 A127667 * A127669 A127670 A127671 KEYWORD nonn,easy,base AUTHOR Wolfdieter Lang Jan 23 2007 EXTENSIONS Edited by Franklin T. Adams-Watters, May 21 2014 STATUS approved

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Last modified March 31 18:30 EDT 2020. Contains 333151 sequences. (Running on oeis4.)