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A075189 Number of distinct primes in the numerator of the 2^n sums generated from the set 1, 1/2, 1/3, ..., 1/n. 6
0, 1, 3, 6, 14, 20, 38, 74, 134, 232, 486, 526, 1078, 2036, 2505, 4762, 9929, 14598, 29831, 31521, 52223, 101123, 207892, 215796, 426772, 836665, 1640357, 1689653, 3401483 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,3

COMMENTS

Every prime is generated eventually. For the largest generated prime, see A075226. For the smallest odd prime not generated, see A075227.

A217712(n) = number of primes occurring exactly once as numerators among the 2^n sums. - Reinhard Zumkeller, Jun 02 2013

LINKS

Table of n, a(n) for n=1..29.

EXAMPLE

a(3) = 3 because 3 sums yield distinct prime numerators: 1+1/2 = 3/2, 1/2+1/3 = 5/6 and 1+1/2+1/3 = 11/6.

MATHEMATICA

Needs["DiscreteMath`Combinatorica`"]; maxN=20; For[lst={}; prms={}; i=0; n=1, n<=maxN, n++, While[i<2^n-1, i++; s=NthSubset[i, Range[n]]; k=Numerator[Plus@@(1/s)]; If[PrimeQ[k], prms=Union[prms, {k}]]]; AppendTo[lst, Length[prms]]]; lst

PROG

(Haskell)

import Data.Ratio ((%), numerator)

import Data.Set (Set, empty, fromList, toList, union, size)

a075189 n = a075189_list !! (n-1)

a075189_list = f 1 empty empty where

   f x s s1 = size s1' : f (x + 1) (s `union` fromList hs) s1' where

     s1' = s1 `union` fromList

           (filter ((== 1) . a010051') $ map numerator hs)

     hs = map (+ 1 % x) $ 0 : toList s

-- Reinhard Zumkeller, May 28 2013

CROSSREFS

Cf. A001008, A075135, A075188, A075226, A075227.

Cf. A010051.

Sequence in context: A175318 A281025 A109757 * A291987 A200823 A093866

Adjacent sequences:  A075186 A075187 A075188 * A075190 A075191 A075192

KEYWORD

nice,nonn

AUTHOR

T. D. Noe, Sep 08 2002

EXTENSIONS

Terms a(21)-a(29) by Reinhard Zumkeller, May 28 2013

STATUS

approved

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Last modified July 28 14:19 EDT 2021. Contains 346335 sequences. (Running on oeis4.)