A274123 -id:A274123 - cp's OEIS frontend

A297408 Where the prime race among 10k+1, ..., 10k+9 changes leader.

2, 13, 157, 193, 347, 383, 587, 673, 907, 1163, 1327, 1483, 1907, 1933, 2897, 4723, 5557, 5573, 6037, 6113, 6637, 6673, 7487, 8273, 8317, 8363, 8387, 8443, 8467, 8573, 8647, 8803, 8837, 8933, 9277, 9293, 10067, 10103, 11897, 11923, 12037, 12073, 12107, 12143

Offset: 1

Sean A. Irvine, Dec 29 2017

nonn

A007355 appears to be an erroneous version of this sequence.

Michael S. Branicky, Table of n, a(n) for n = 1..10000
Robert G. Wilson v, Letter to N. J. A. Sloane, Aug. 1993

Cf. A007352, A007350, A007353, A007354, A274121, A274122, A274123, A297406, A297407, A297408, A297410, A297411.

a297408(limit)={my(v=vector(10),vm=0,ivm=0,imv); forprime(p=2,limit,my(m=p%10);v[m]++;my(mv=vecmax(v,&imv));if(mv>vm,if(imv!=ivm,print1(p,", "); ivm=imv);vm=mv))};
a297408(12500) \\ Hugo Pfoertner, Jul 25 2021

from sympy import nextprime
from itertools import islice
def agen():
    c, p, leader = [0 for i in range(10)], 1, None
    while True:
        p = nextprime(p); last = p%10; c[last] += 1; m = max(c)
        if c.count(m) == 1 and c.index(m) == last and last != leader:
            yield p; leader = last
print(list(islice(agen(), 44))) # Michael S. Branicky, Dec 20 2022

A274121 The gap prime(n+1) - prime(n) occurs for the a(n)-th time.

Original entry on oeis.org

1, 1, 2, 1, 3, 2, 4, 3, 1, 5, 2, 4, 6, 5, 3, 4, 7, 5, 6, 8, 6, 7, 7, 1, 8, 9, 9, 10, 10, 1, 11, 8, 11, 1, 12, 9, 10, 12, 11, 12, 13, 2, 14, 13, 15, 1, 2, 14, 16, 15, 13, 17, 3, 14, 15, 16, 18, 17, 16, 19, 4, 2, 17, 20, 18, 3, 18, 5, 21, 19, 19, 2, 20, 21, 20, 22, 3, 21, 4, 6, 22, 7, 23, 23, 22, 24, 5, 23, 24, 24, 3, 6

Offset: 1

David A. Corneth, Jun 10 2016

nonn

Terms of this sequence grow without bound; any even number occurs in this sequence. Zhang proved that there are infinitely many primes 4680 apart from each other (see link "Bounded gaps between primes").
For a conjectured count of gap n below x, see link Polignac's conjecture.
Polignac's conjecture states that "For any positive even number n, there are infinitely many prime gaps of size n.". By this conjecture, every positive apppears infinitely many times in this sequence (see link "Polignac's conjecture").

			(p, g) denotes a prime p and the gap up to the next prime. So p + g is the next prime after p. These pairs start (2, 1), (3, 2), (5, 2), (7, 4), (11, 2). From here we see that:
- the gap after the first prime, 1 occurs for the first time, so a(1) = 1.
- the gap after the second prime, 2, occurs for the first time, so a(2) = 1.
- the gap after the third prime, 2, occurs for the second time, so a(3) = 2.
- the gap after the fourth prime, 4, occurs for the first time, so a(4) = 1.
- the gap after the fifth prime, 2, occurs for the third time, so a(5) = 3.

David A. Corneth, Table of n, a(n) for n = 1..100021 (all terms up to 1300000)
David A. Corneth, PARI program
PolyMath, Bounded gaps between primes.
Wikipedia, Polignac's conjecture.

Cf. A000230, A005597, A001359, A029710, A274122, A274123.

\\ See link by name "PARI program" for an extended version with comments.
upto(n) = {my(gapcount=List(), freqgap = List([1])); n = max(n, 3); forprime(i=3,n,
g = nextprime(i+1) - i; for(i=#gapcount+1, g\2, listput(gapcount,0));  gapcount[g\2]++; listput(freqgap,gapcount[g\2]));freqgap} \\ David A. Corneth, Jun 28 2016

a(primepi(A000230(n))) = 1.
a(primepi(A001359(n))) = n.
a(primepi(A029710(n))) = n.

A274122 Let F(g,p) be the frequency of g up to prime nextprime(p+1). Primes p such that F(2,p) = F(4,p) and g = 2 or 4.

Original entry on oeis.org

97, 103, 109, 137, 163, 397, 457, 463, 569, 613, 739, 821, 827, 857, 2111, 2137, 2203, 2239, 2269, 2309, 2347, 2381, 4127, 4217, 8803, 9337, 12487, 12739, 12889, 12917, 19991, 20021, 20717, 20747, 20771, 20959, 21187, 21313, 21319, 21379, 21599, 21613, 21817, 21859, 22037, 22091, 22129, 22157, 22271, 22277, 22481, 22567

Offset: 1

David A. Corneth, Jun 10 2016

nonn

Gaps 2 and 4 are conjectured to occur roughly equally often and each occur infinitely often (see link "Polignac's conjecture").

Conjecture: This sequence is infinite.

			Up to 89, gaps 2 and 4 occur respectively 8 and 7 times. After 97 (the next prime after 89), there is a gap of 4. So up to 97, gaps 2 and 4 occur the same number of times and the gap after 97 is 2 or 4, so 97 is in the sequence.

David A. Corneth, Table of n, a(n) for n = 1..862 (all terms up to 10^8)
David A. Corneth, PARI program
PolyMath, Bounded gaps between primes.
Wikipedia, Polignac's conjecture.

Cf. A007351, A274121, A274123.

\\ See link by name "PARI program" for an extended version with comments.
upto(n) = {my(gapcount=List(),is24 = List()); n=max(n,3); forprime(i=3,n, g = nextprime(i+1) - i; for(i=#gapcount+1,g\2, listput(gapcount, 0));
gapcount[g\2]++; if(g<6&&#gapcount>1,if(gapcount[1]==gapcount[2],listput(is24,i)))); is24} \\ David A. Corneth, Jun 28 2016

A276176 Consider the race between primes, semiprimes, 3-almost primes, ... k-almost primes; sequence indicates when one overtakes another to give a new race leader.

Original entry on oeis.org

2, 26, 31, 34, 15526, 151165506068, 151165506073, 151165506089, 151165506093, 151165506295, 151165506410, 151165506518, 151165506526, 151165506658, 151165506665, 151165506711, 151165506819, 151165506970, 151165506994, 151165507256, 151165507259, 151165507265

Offset: 1

Jonathan Vos Post and Robert G. Wilson v, Oct 09 2016

A "k-almost prime" is a number which is the product of exactly k primes.
Let pi_k(n) be the number of k-almost primes less than or equal to n. In 1909, on page 211 of the Handbuch, Edmund G. H. Landau stated that pi_k(n) ~ (n/log n)*(log log n^(k-1))/(k-1)! for all k >= 0.
Because of this fact, eventually the semiprimes will outnumber the primes; they do starting at 34. Likewise the 3-almost primes will outnumber the semiprimes and they do starting at 15526.
The terms from a(6) = 151165506068 to a(170) = 151165607026 correspond to counts of 4-almost and 3-almost primes overtaking each other multiple times. - Giovanni Resta, Aug 17 2018

			a(1) = 2 since beginning with the natural numbers (A000027) the race is even with no group in the lead. But at 2, we encounter our first member (1 is unity and is not a member of any group here) which is a prime and therefore the primes take the lead with 2.
a(2) = 34 which is a semiprime. pi_1(34) = 11 and pi_2(34) = 12. This is the first time that the semiprimes overtake the primes.

Edmund Georg Hermann Landau, Handbuch der Lehre von der Verteilung der Primzahlen, Band I, B. G. Teubner, Leipzig u. Berlin, or Chelsea Publishing, NY 1953, or Vol. 1, Teubner, Leipzig; third edition: Chelsea, New York 1974.

Giovanni Resta, Table of n, a(n) for n = 1..170
Edmund Georg Hermann Landau, Handbuch der Lehre von der Verteilung der Primzahlen, page 211, (4); University of Michigan Historical Math Collection.

Cf. A243906, A273381, A274123, A358677, A359242 (restricted to squarefree numbers).

k = 1; lst = {}; tf = 0; p1 = 0; p2 = 0; While[k < 100001, If[PrimeOmega@k == 1, p1++]; If[PrimeOmega@k == 2, p2++]; If[p1 > p2 && tf == 0, tf++; AppendTo[lst, k]]; If[p2 > p1 && tf == 1, tf--; AppendTo[lst, k]]; k++]; lst
(* cross check using *) AlmostPrimePi[k_Integer, n_] := Module[{a, i}, a[0] = 1; If[k == 1, PrimePi[n], Sum[PrimePi[n/Times @@ Prime[ Array[a, k - 1]]] - a[k - 1] + 1, Evaluate[ Sequence @@ Table[{a[i], a[i - 1], PrimePi[(n/Times @@ Prime[Array[a, i - 1]])^(1/(k - i + 1))]}, {i, k - 1}]] ]]]; (* Eric W. Weisstein, Feb 07 2006 *)
(* as an example *) AlmostPrimePi[2, 15526] => 3986 whereas AlmostPrimePi[3, 15526] => 3987.

It seems plausible that 0.8 * log(A284411(m) - 1) <= log(a(n)) <= log(A284411(m)) in the instances where the overtaking concerns m-almost-primes and (m-1)-almost-primes. - Peter Munn, Aug 03 2023

a(6)-a(22) from Giovanni Resta, Aug 17 2018

Name clarified by Peter Munn, Dec 31 2022

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A297408 Where the prime race among 10k+1, ..., 10k+9 changes leader.

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A274121 The gap prime(n+1) - prime(n) occurs for the a(n)-th time.

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A274122 Let F(g,p) be the frequency of g up to prime nextprime(p+1). Primes p such that F(2,p) = F(4,p) and g = 2 or 4.

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A276176 Consider the race between primes, semiprimes, 3-almost primes, ... k-almost primes; sequence indicates when one overtakes another to give a new race leader.

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