A075227 -id:A075227 - cp's OEIS frontend

A075188 Number of times that the numerator of a sum generated from the set 1, 1/2, 1/3,..., 1/n is prime.

0, 1, 3, 9, 19, 43, 79, 162, 307, 607, 1075, 2186, 3872, 7573, 15101, 29139, 52295, 104953, 189915, 379275, 754081, 1462115, 2675851, 5351541, 10254019, 19987942, 38901233, 77620568, 144021667, 288428481, 537642772, 1056802340, 2113152353, 4138261885

Offset: 1

T. D. Noe, Sep 08 2002

Note that for each n there are only 2^(n-1) new sums to consider. Surprisingly, nearly half of the sums have a prime numerator. For the number of distinct primes, see A075189. 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. - Reinhard Zumkeller, Jun 02 2013

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

Cf. A001008, A075135, A075189, A075226, A075227.

Cf. A010051.

import Data.Ratio (numerator)
a075188 n = a075188_list !! (n-1)
a075188_list = f 1 [] where
   f x hs = (length $ filter ((== 1) . a010051') (map numerator hs')) :
            f (x + 1) hs' where hs' = hs ++ map (+ recip x) (0 : hs)
-- Reinhard Zumkeller, May 28 2013

Needs["DiscreteMath`Combinatorica`"]; maxN=20; For[cnt=0; lst={}; 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], cnt++ ]]; AppendTo[lst, cnt]]; lst

a(21)-a(25) by Reinhard Zumkeller, May 28 2013
a(26)-a(31) from Chai Wah Wu, Feb 14 2022
a(32)-a(34) from Sean A. Irvine, Feb 10 2025

A075189 Number of distinct primes in the numerator of the 2^n sums generated from the set 1, 1/2, 1/3, ..., 1/n.

Original entry on oeis.org

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, 3471770, 6868800, 13470379, 23182192, 45792615, 47136366

Offset: 1

T. D. Noe, Sep 08 2002

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

			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.

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

Cf. A010051.

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

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

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

a(30)-a(35) from Sean A. Irvine, Feb 10 2025

A075226 Largest prime in the numerator of the 2^n sums generated from the set 1, 1/2, 1/3,..., 1/n.

Original entry on oeis.org

3, 11, 19, 137, 137, 1019, 2143, 7129, 7129, 78167, 81401, 1085933, 1111673, 1165727, 2364487, 41325407, 41325407, 796326437, 809074601, 812400209, 822981689, 19174119571, 19652175721, 99554817251, 100483070801

Offset: 2

T. D. Noe, Sep 08 2002

For the smallest odd prime not generated, see A075227. For information about how often the numerator of these sums is prime, see A075188 and A075189. The Mathematica program also prints the subset that yields the largest prime. For n <=20, the largest prime occurs in a sum of n-2, n-1, or n reciprocals.

			a(3) =11 because 11 is largest prime numerator in the three sums that yield primes: 1+1/2 = 3/2, 1/2+1/3 = 5/6 and 1+1/2+1/3 = 11/6.

Chai Wah Wu, Table of n, a(n) for n = 2..441 (terms 2..100 from Martin Fuller)
Martin Fuller, PARI program

Cf. A001008, A075135, A075188, A075189, A075227, A010051, A217712.

import Data.Ratio (numerator)
a075226 n = a075226_list !! (n-1)
a075226_list = f 2 [recip 1] where
   f x hs = (maximum $ filter ((== 1) . a010051') (map numerator hs')) :
            f (x + 1) hs' where hs' = hs ++ map (+ recip x) hs
-- Reinhard Zumkeller, May 28 2013

Needs["DiscreteMath`Combinatorica`"]; maxN=20; For[t={}; lst={}; mx=0; i=0; n=2, n<=maxN, n++, While[i<2^n-1, i++; s=NthSubset[i, Range[n]]; k=Numerator[Plus@@(1/s)]; If[PrimeQ[k], If[k>mx, t=s]; mx=Max[mx, k]]]; Print[n, " ", t]; AppendTo[lst, mx]]; lst
Table[Max[Select[Numerator[Total/@Subsets[1/Range[n],{2,2^n}]],PrimeQ]],{n,2,30}] (* The program will take a long time to run. *) (* Harvey P. Dale, Jan 08 2019 *)

PARI
```
See Fuller link.
```

from math import gcd, lcm
from itertools import combinations
from sympy import isprime
def A075226(n):
    m = lcm(*range(1,n+1))
    c, mlist = 0, tuple(m//i for i in range(1,n+1))
    for l in range(n,-1,-1):
        if sum(mlist[:l]) < c:
            break
        for p in combinations(mlist,l):
            s = sum(p)
            s //= gcd(s,m)
            if s > c and isprime(s):
                c = s
    return c # Chai Wah Wu, Feb 14 2022

More terms from Martin Fuller, Jan 19 2008

A217712 Number of primes occurring exactly once as numerators in sums generated from the set 1, 1/2, 1/3,..., 1/n.

Original entry on oeis.org

0, 1, 3, 3, 11, 13, 27, 54, 106, 168, 378, 142, 733, 1597, 1283, 3418, 8204, 10112, 24644, 7829, 32866, 78136, 178741, 37002, 256392, 650596, 1402914, 286854, 2053463

Offset: 1

Reinhard Zumkeller, Jun 02 2013

nonn

For information about how often the numerator of the generated sums is prime, see A075188 and A075189; for the largest generated prime, see A075226; for the smallest odd prime not generated, see A075227.

			For n=3 there are the following fractions as sums of 1, 1/2 and 1/3:
{1/3, 1/2, 5/6, 1, 4/3, 3/2, 11/6}, three numerators are prime and they occur exactly once, therefore a(3) = A075188(3) = A075189(3) = #{3, 5, 11} = 3;
n=4: adding 1/4 to the previous fractions gives together: 1/4, 1/3, 1/2, 1/3+1/4=7/12, 1/2+1/4=3/4, 5/6, 1, 5/6+1/4=13/12, 1+1/4=5/4, 4/3, 3/2, 4/3+1/4=19/12, 3/2+1/4=7/4, 11/6 and 11/6+1/4=25/12:
A075188(4) = #{7/12, 3/4, 5/6, 13/12, 5/4, 3/2, 19/12, 7/4, 11/6} = 9,
A075189(4) = #{3, 5, 7, 11, 13, 19} = 6,
a(4) = #{11, 13, 19} = 3.

Cf. A010051.

import Data.Ratio ((%), numerator)
import Data.Set (Set, empty, fromList, toList, union, size)
import Data.Set (member, delete, insert)
a217712 n = a217712_list !! (n-1)
a217712_list = f 1 empty empty where
   f x s s1 = size s1' : f (x + 1) (s `union` fromList hs) s1' where
     s1' = g s1 $ filter ((== 1) . a010051') $ map numerator hs
     g v []                    = v
     g v (w:ws) | w `member` v = g (delete w v) ws
                | otherwise    = g (insert w v) ws
     hs = map (+ 1 % x) $ 0 : toList s

A256223 Smallest Fibonacci number not occurring in the numerator of the 2^n sums generated from the set 1, 1/2, 1/3,..., 1/n.

Original entry on oeis.org

1, 2, 2, 2, 2, 2, 21, 21, 21, 21, 34, 34, 144, 144, 144, 144, 144, 144, 144, 144, 144, 144, 987, 987, 987, 987, 987, 987, 987, 987, 987, 987, 46368, 46368, 46368, 46368, 46368, 46368, 46368, 46368, 46368, 832040, 832040, 832040, 832040, 832040, 832040, 832040

Offset: 0

Michel Lagneau, Mar 19 2015

nonn

The largest prime generated is given in A256222.

			a(3) = 2 because we obtain 5 following subsets {1}, {1/2}, {1/3}, {1, 1/2} and {1/2, 1/3} having 5 sums with Fibonacci numerators: 1, 1, 1, 1+1/2 = 3/2 and 1/2+1/3 = 5/6. Then, 2 is the smallest Fibonacci number not occurring in the numerator of the previous sums.

Hiroaki Yamanouchi, Table of n, a(n) for n = 0..50

Cf. A000045, A075227, A256220, A256221, A256222.

<<"DiscreteMath`Combinatorica`"; maxN=23; For[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[IntegerQ[Sqrt[5*k^2+4]]||IntegerQ[Sqrt[5*k^2-4]], AppendTo[prms, k]]]; prms=Union[prms]; j=2; While[MemberQ[prms, Fibonacci[j]], j++ ]; Print[Fibonacci[j]]]

a(0) prepended and a(24)-a(47) added by Hiroaki Yamanouchi, Mar 30 2015

cp's OEIS Frontend

A075188 Number of times that the numerator of a sum generated from the set 1, 1/2, 1/3,..., 1/n is prime.

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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.

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A075226 Largest prime in the numerator of the 2^n sums generated from the set 1, 1/2, 1/3,..., 1/n.

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A217712 Number of primes occurring exactly once as numerators in sums generated from the set 1, 1/2, 1/3,..., 1/n.

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Haskell

A256223 Smallest Fibonacci number not occurring in the numerator of the 2^n sums generated from the set 1, 1/2, 1/3,..., 1/n.

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Programs

Mathematica

Extensions