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 A182590 Number of distinct prime factors of 2^n - 1 of the form k*n + 1. 13
 1, 1, 1, 1, 1, 1, 1, 1, 2, 2, 1, 1, 2, 2, 2, 1, 2, 1, 1, 2, 3, 2, 1, 2, 2, 1, 2, 3, 3, 1, 2, 1, 2, 2, 3, 2, 2, 3, 2, 2, 4, 3, 3, 2, 3, 3, 3, 1, 4, 4, 3, 3, 2, 3, 2, 3, 5, 2, 2, 1, 2, 3, 4, 2, 3, 2, 3, 1, 4, 3, 3, 3, 4, 5, 3, 1, 5, 3, 2, 3, 4, 2, 3, 2, 4, 3 (list; graph; refs; listen; history; text; internal format)
 OFFSET 2,9 COMMENTS From Thomas Ordowski, Sep 08 2017: (Start) By Bang's theorem, a(n) > 0 for all n > 1, see A186522. Primes p such that a(p) = 1 are the Mersenne exponents A000043. Composite numbers m for which a(m) = 1 are A292079. a(n) >= A086251(n), where equality is for all prime numbers and for some composite numbers (among others for all odd prime powers p^k with k > 1). Theorem: if n is prime, then a(n) = A046800(n). Conjecture: if a(n) = A046800(n), then n is prime. Problem: is a(n) < A046800(n) for every composite n? (End) LINKS Charles R Greathouse IV, Table of n, a(n) for n = 2..1200 (terms 2..200 from Seppo Mustonen, terms 201..786 from Michel Marcus) S. Mustonen, On prime factors of numbers m^n+-1, 2010. Seppo Mustonen, On prime factors of numbers m^n+-1 [Local copy] EXAMPLE For n=10 the prime factors of 2^n - 1 = 1023 are 3, 11 and 31, and 11 = n+1, 31 = 3n+1. Thus a(10)=2. MATHEMATICA m = 2; n = 2; nmax = 200; While[n <= nmax, {l = FactorInteger[m^n - 1]; s = 0;      For[i = 1, i <= Length[l],       i++, {p = l[[i, 1]];        If[IntegerQ[(p - 1)/n] == True, s = s + l[[i, 2]]]; }];      a[n] = s; } n++; ]; Table[a[n], {n, 2, nmax}] PROG (PARI) a(n) = my(f = factor(2^n-1)); sum(k=1, #f~, ((f[k, 1]-1) % n)==0); \\ Michel Marcus, Sep 10 2017 CROSSREFS Cf. A046800, A086251, A186522. Sequence in context: A220163 A102715 A254687 * A047846 A212632 A025885 Adjacent sequences:  A182587 A182588 A182589 * A182591 A182592 A182593 KEYWORD nonn AUTHOR Seppo Mustonen, Nov 22 2010 EXTENSIONS Name edited by Thomas Ordowski, Sep 19 2017. STATUS approved

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Last modified June 4 01:54 EDT 2020. Contains 334812 sequences. (Running on oeis4.)