A213980 Let n = prime(1)^c_1 * prime(2)^c_2 * ... * prime(k)^c_k be the prime factorization of n. Set f(n) = n - 1 + c_1 + c_2 + ... + c_k and f_i, i>=0 (f_0(n) = n, f_1=f) is i-th iteration of f. a(n) is the minimal i such that f_i(n) is prime.
0, 0, 1, 0, 1, 0, 2, 2, 1, 0, 4, 0, 3, 2, 1, 0, 3, 0, 2, 2, 1, 0, 2, 3, 2, 1, 6, 0, 5, 0, 4, 6, 5, 4, 3, 0, 3, 2, 1, 0, 3, 0, 2, 1, 1, 0, 5, 6, 5, 5, 4, 0, 3, 2, 1, 2, 1, 0, 18, 0, 18, 17, 15, 16, 15, 0, 14, 14, 13, 0, 12, 0, 13, 12, 11, 11, 10, 0, 9, 9, 1, 0, 8, 9, 8, 7, 6, 0, 5, 5, 4, 4, 3, 2, 1, 0, 2, 1, 1, 0, 2, 0, 1, 1, 1, 0, 16, 0
Offset: 2
Keywords
Examples
f_1(12) = 12+2+1-1 = 14, f_1(14) = 14+1+1-1 = 15, f_1(15) = 15+1+1-1 = 16, f_1(16) = 16+4-1 = 19. Since to get to a prime we used 4 iterations, a(12)=4.
Links
- Amiram Eldar, Table of n, a(n) for n = 2..10000
Crossrefs
f_1 is A222312.
Programs
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Mathematica
a[n_] := Block[{x = n, c = 0}, While[! PrimeQ[x], x = x-1 + Total[Last /@ FactorInteger[x]]; c++]; c]; a/@Range[2,109] (* Giovanni Resta, Feb 16 2013 *)
Extensions
a(81) corrected by Giovanni Resta, Feb 16 2013
Comments