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A080359 The smallest integer x > 0 such that the number of primes in (x/2, x] equals n. 43
2, 3, 13, 19, 31, 43, 53, 61, 71, 73, 101, 103, 109, 113, 139, 157, 173, 181, 191, 193, 199, 239, 241, 251, 269, 271, 283, 293, 313, 349, 353, 373, 379, 409, 419, 421, 433, 439, 443, 463, 491, 499, 509, 523, 577, 593, 599, 601, 607, 613, 619, 647, 653, 659 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

a(n) is the same as: Smallest integer x > 0 such that the number of unitary-prime-divisors of x! equals n.

Let p_n be the n-th prime. If p_n>3 is in the sequence, then all integers (p_n-1)/2, (p_n-3)/2, ..., (p_(n-1)+1)/2 are composite numbers. - Vladimir Shevelev, Aug 12 2009

For n >= 3, denote by q(n) the prime which is the nearest from the left to a(n)/2. Then there exists a prime between 2q(n) and a(n). The converse, generally speaking, is not true; i.e., there exist primes that are outside the sequence, but possess such property (e.g., 131). - Vladimir Shevelev, Aug 14 2009

See sequence A164958 for a generalization. - Vladimir Shevelev, Sep 02 2009

a(n) is the n-th Labos prime.

LINKS

Daniel Forgues, Table of n, a(n) for n=1..4460

N. Amersi, O. Beckwith, S. J. Miller, R. Ronan, J. Sondow, Generalized Ramanujan primes, arXiv:1108.0475 [math.NT], 2011.

N. Amersi, O. Beckwith, S. J. Miller, R. Ronan, J. Sondow, Generalized Ramanujan primes, Combinatorial and Additive Number Theory, Springer Proc. in Math. & Stat., CANT 2011 and 2012, Vol. 101 (2014), 1-13

V. Shevelev, On critical small intervals containing primes, arXiv:0908.2319 [math.NT], 2009.

V. Shevelev, Ramanujan and Labos primes, their generalizations and classifications of primes, arXiv:0909.0715 [math.NT], 2009-2011.

V. Shevelev, Ramanujan and Labos primes, their generalizations, and classifications of primes, J. Integer Seq. 15 (2012) Article 12.5.4

J. Sondow, MathWorld: Ramanujan Prime

J. Sondow, Ramanujan primes and Bertrand's postulate, arXiv:0907.5232 [math.NT], 2009-2010.

J. Sondow, Ramanujan primes and Bertrand's postulate, Amer. Math. Monthly, 116 (2009), 630-635.

FORMULA

a(n) = Min{x; Pi[x]-Pi[x/2]=n} = Min{x; A056171(x)=n}=Min{x; A056169(n!)=n}; where Pi()=A000720().

a(n) <= A193507(n) (cf. A194186). - Vladimir Shevelev, Aug 18 2011

EXAMPLE

n=5: in 31! five unitary-prime-divisors appear (firstly): {17,19,23,29,31}, while other primes {2,3,5,7,11,13} are at least squared. Thus a(5)=31.

Consider a(9)=71. Then the nearest prime < 71/2 is q(9)=31, and between 2q(9) and a(9), i.e., between 62 and 71 there exists a prime (67). - Vladimir Shevelev, Aug 14 2009

MATHEMATICA

nn=1000; t=Table[0, {nn+1}]; s=0; Do[If[PrimeQ[k], s++]; If[PrimeQ[k/2], s--]; If[s<=nn && t[[s+1]]==0, t[[s+1]]=k], {k, Prime[3*nn]}]; Rest[t]

(* Second program: *)

a[1] = 2; a[n_] := a[n] = Module[{x = a[n-1]}, While[(PrimePi[x]-PrimePi[Quotient[x, 2]]) != n, x++ ]; x]; Array[a, 54] (* Jean-Fran├žois Alcover, Sep 14 2018 *)

PROG

(PARI) a(n) = {my(x = 1); while ((primepi(x) - primepi(x\2)) != n, x++; ); x; } \\ Michel Marcus, Jan 15 2014

(Sage)

def A():

    i = 0; n = 1

    while True:

        p = prime_pi(i) - prime_pi(i//2)

        if p == n:

            yield i

            n += 1

        i += 1

A080359 = A()

[A080359.next() for n in range(54)] # Peter Luschny, Sep 03 2014

CROSSREFS

Cf. A056171, A056169, A000720, A000142.

Cf. A104272 (Ramanujan primes).

Cf. A060756, A080360 (largest integer x with n primes in (x/2,x]).

Cf. A164554, A164288, A164333, A164294, A164372, A164371, A212493, A212541.

Sequence in context: A254462 A275030 A194598 * A193507 A103087 A302485

Adjacent sequences:  A080356 A080357 A080358 * A080360 A080361 A080362

KEYWORD

nonn

AUTHOR

Labos Elemer, Feb 21 2003

EXTENSIONS

Definition corrected by Jonathan Sondow, Aug 10 2008

Shrunk title and moved part of title to comments by John W. Nicholson, Sep 18 2011

STATUS

approved

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Last modified November 12 09:29 EST 2019. Contains 329054 sequences. (Running on oeis4.)