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 A007875 Number of ways of writing n as p*q, with p <= q, gcd(p, q) = 1. 13
 1, 1, 1, 1, 1, 2, 1, 1, 1, 2, 1, 2, 1, 2, 2, 1, 1, 2, 1, 2, 2, 2, 1, 2, 1, 2, 1, 2, 1, 4, 1, 1, 2, 2, 2, 2, 1, 2, 2, 2, 1, 4, 1, 2, 2, 2, 1, 2, 1, 2, 2, 2, 1, 2, 2, 2, 2, 2, 1, 4, 1, 2, 2, 1, 2, 4, 1, 2, 2, 4, 1, 2, 1, 2, 2, 2, 2, 4, 1, 2, 1, 2, 1, 4, 2, 2, 2, 2, 1, 4 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,6 COMMENTS a(n), n >= 2, is the number of divisor products in the numerator as well as denominator of the unique representation of n in terms of divisor products. See the W. Lang link under A007955, where a(n)=l(n) in Table 1. - Wolfdieter Lang, Feb 08 2011 Record values are the binary powers, occurring at primorial positions except at 2: a(A002110(0))=A000079(0), a(A002110(n+1))=A000079(n) for n > 0. - Reinhard Zumkeller, Aug 24 2011 For n > 1: a(n) = (A000005(n) - A048105(n)) / 2; number of ones in row n of triangle in A225817. - Reinhard Zumkeller, Jul 30 2013 LINKS T. D. Noe, Table of n, a(n) for n = 1..1000 Larry Bates, Peter Gibson, A geometry where everything is better than nice, arXiv:1603.06622 [math.DG], (21-March-2016); see page 2. FORMULA a(n) = (1/2)*Sum_{ d divides n } abs(mu(d)) = 2^(A001221(n)-1) = A034444(n)/2, n > 1. - Vladeta Jovovic, Jan 25 2002 a(n) = phi(2^omega(n)) = A000010(2^A001221(n)). - Enrique Pérez Herrero, Apr 10 2012 MAPLE A007875 := proc(n)     if n = 1 then         1;     else         2^(A001221(n)-1) ;     end if; end proc: # R. J. Mathar, May 28 2016 MATHEMATICA a[n_] := With[{r = Reduce[1 <= p <= q <= n && n == p*q && GCD[p, q] == 1, {p, q}, Integers]}, If[Head[r] === And, 1, Length[r]]]; Table[a[n], {n, 1, 90}] (* Jean-François Alcover, Nov 02 2011 *) a[n_] := EulerPhi[2^PrimeNu[n]]; Array[a, 105] (* Robert G. Wilson v, Apr 10 2012 *) a[n_] := Sum[If[Mod[n, k] == 0, Re[Sqrt[MoebiusMu[k]]], 0], {k, 1, n}] (* Mats Granvik, Aug 10 2018 *) PROG (Haskell) a007875 = length . filter (> 0) . a225817_row -- Reinhard Zumkeller, Jul 30 2013, Aug 24 2011. (PARI) a(n)=ceil((1<

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Last modified August 15 09:12 EDT 2018. Contains 313756 sequences. (Running on oeis4.)