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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
OFFSET
0,1
COMMENTS
Is this sequence infinite? - David A. Corneth, Aug 17 2018
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: A253203 A338151 A255642 * A153698 A168203 A248310
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