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A246782 Numbers n such that A182134(n)=2, i.e., there exist only two primes p with prime(n) < p < prime(n)^(1+1/n). 7
5, 6, 7, 9, 10, 11, 14, 15, 22, 23, 28, 29, 30, 45, 46, 61, 66, 216, 217, 367, 3793, 1319945, 1576499, 8040877, 17567976, 44405858, 445538764, 1478061204, 3643075047, 17440041685, 190836014732, 714573709895, 714573709896 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

Firoozbakht's conjecture says that for every n, there exists at least one prime p such that prime(n) < p < prime(n)^(1+1/n).

Let A(m) = {n | A182134(n) = m} where A182134(n) = #{p | p is prime and prime(n) < p < prime(n)^(1+1/n)}. This sequence gives the terms of A(2) and the sequence A246781 gives the terms of A(3).

The only known indices n for which A182134(n) = 1 are {1, 2, 3, 4, 8}.  It is conjectured that this is the complete set A(1).

Conjecture: For all m, where m is greater than one, A(m) is an infinite set.

a1 = 49749629143524, a2 = 1475067052906944 and a3 = 1475067052906945 are three large terms of the sequence. It is interesting that a3 - a2 = 1.

Conjecture: The sequence is infinite.

Next term is greater than 25000000.

a(34) > 10^12. - Robert Price, Nov 01 2014

The conjecture that A(1)={1, 2, 3, 4, 8} holds through 10^12. - Robert Price, Nov 01 2014

LINKS

Table of n, a(n) for n=1..33.

A. Kourbatov, Verification of the Firoozbakht conjecture for primes up to four quintillion, arXiv:1503.01744 [math.NT], 2015

Wikipedia, Firoozbakht's conjecture

EXAMPLE

5 is in the sequence since there exists only two primes p,  prime(5) < p < prime(5)^(1+1/5). Note that prime(5) = 11, 11^(1+1/5)) ~ 17.77 and 11 < 13 < 17 < 17.77.

MATHEMATICA

np[n_]:=(a = Prime[n]; b = a^(1 + 1/n); Length[Select[Range[a+1, b], PrimeQ]]); Do[If[np[n] == 2, Print[n]], {n, 25000000}]

PROG

(PARI) for(n=1, 9e9, 2==primepi(prime(n)^(1+1/n))-n&&print1(n", ") \\ M. F. Hasler, Nov 03 2014

(Haskell)

a246782 n = a246782_list !! (n-1)

a246782_list = filter ((== 2) . a182134) [1..]

-- Reinhard Zumkeller, Nov 17 2014

CROSSREFS

Cf. A000040, A002386, A005669, A182134, A246781, A249566.

Sequence in context: A191850 A066263 A178096 * A089192 A279195 A232710

Adjacent sequences:  A246779 A246780 A246781 * A246783 A246784 A246785

KEYWORD

nonn,more

AUTHOR

Farideh Firoozbakht, Oct 12 2014

EXTENSIONS

a(26)-a(27) from Robert Price, Oct 24 2014

a(28)-a(33) from Robert Price, Nov 01 2014

STATUS

approved

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Last modified October 22 23:18 EDT 2018. Contains 316518 sequences. (Running on oeis4.)