A304457 Least semiprime of a run of exactly n odd semiprimes.

9, 15, 87, 49, 235, 403, 1241, 1383, 1047, 3341, 4359, 9663, 11499, 14163, 5947, 11183, 19103, 45151, 50943, 31367, 173747, 306003, 215021, 191303, 86663, 62797, 362609, 425405, 346727, 531247, 676067, 721311, 793469, 741899, 1637447, 2896177, 311843, 4477649, 7702927, 1208153

Offset

1,1

Comments

Runs of semiprimes of the same parity: {4, 6}, {9}, {10, 14}, {15, 21}, {22}, {25}, {26}, {33}, {34}, {35}, {38}, {39}, {46}, {49, 51, 55, 57}, {58, 62}, {65, 69}, {74}, {77}, {82}, {85}, {86}, {87, 91, 93}, {94}, ....

a(108) = 85305236689; a(101-107), if any, should be > 116820000000. - Zak Seidov, Aug 26 2021

Links

Silvester Resnik, Table of n, a(n) for n = 1..100

Example

a(1) = 9 since it is the first occurrence of a single odd semiprime;
a(2) = 15 since 15 is the first of two consecutive odd semiprimes in the sequence of semiprimes;
a(3) = 87 since 87 is the first of exactly three consecutive odd semiprimes;
a(4) = 49 since 49 is the first of exactly four consecutive odd semiprimes; etc.

Mathematica

SplitBy[ Select[ Range@ 100, PrimeOmega@# == 2 &], Mod[#, 2] &] (* to view the runs of semiprimes of the same parity *)

Crossrefs

Cf. A001358, A304456, A304458.

Sequence in context: A351499 A037378 A100663 * A219129 A226927 A152219

Adjacent sequences: A304454 A304455 A304456 * A304458 A304459 A304460

Keyword

easy,nonn

Author

Zak Seidov, Silvester Resnik and Robert G. Wilson v, May 12 2018

Status

approved