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A088701 Smallest semiprime such that n primes will follow until the next semiprime. 2
9, 4, 10, 95, 818, 2681, 16053, 35981, 797542, 1069541, 340894, 6974465, 3586843, 2070050198, 5238280946, 14499777101, 2601693504238, 7472089131123 (list; graph; refs; listen; history; text; internal format)
OFFSET

0,1

COMMENTS

Is this sequence infinite? - David A. Corneth, Aug 17 2018

LINKS

Table of n, a(n) for n=0..17.

FORMULA

A088700(a(n)) = n and A088700(k) <> n for 1 <= k < a(n).

MATHEMATICA

om = Array[PrimeOmega, 1100000]; sp = Flatten@ Position[om, 2]; Table[ sp[[ SelectFirst[ Range[Length[sp] - 1], Count[Take[om, {sp[[#]], sp[[# + 1]]}], x_ /; x == 1] == j &, 0]]], {j, 0, 10}] (* Giovanni Resta, Aug 16 2018 *)

PROG

(Perl) use ntheory ":all";

my($l, $nextn, @C)=(4, 0);

forcomposites {

  if (is_semiprime($_)) {

    my $c = prime_count($l+1, $_-1);

    if (!defined $C[$c]) {

      $C[$c] = $l;

      while (defined $C[$nextn]) { print "$nextn $C[$nextn]\n"; $nextn++; }

    }

    $l = $_;

  }

} 5, 1e7;  # Dana Jacobsen, Aug 16 2018

CROSSREFS

Cf. A214520 (primes that are the only prime between consecutive semiprimes).

Sequence in context: A154201 A253203 A255642 * A153698 A168203 A248310

Adjacent sequences:  A088698 A088699 A088700 * A088702 A088703 A088704

KEYWORD

nonn,more

AUTHOR

Reinhard Zumkeller, Oct 08 2003

EXTENSIONS

a(11)-a(15) from Donovan Johnson, Mar 14 2010

a(16) from Giovanni Resta, Aug 17 2018

a(17) from Giovanni Resta, Aug 18 2018

STATUS

approved

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Last modified August 22 11:34 EDT 2019. Contains 326176 sequences. (Running on oeis4.)