A305755 Odd semiprimes sandwiched by even semiprimes.

9, 25, 33, 35, 39, 77, 85, 95, 141, 155, 201, 259, 355, 361, 395, 481, 559, 565, 623, 763, 799, 841, 865, 879, 923, 933, 979, 1003, 1043, 1177, 1199, 1219, 1227, 1285, 1293, 1299, 1437, 1441, 1517, 1541, 1603, 1643, 1655, 1717, 1769, 1865, 2005, 2041, 2095, 2101, 2123, 2127, 2183, 2245

Offset

1,1

Comments

Odd semiprimes sp(k) = A001358(k) such that sp(k-1) and sp(k+1) are even. Values of k: 3, 9, 11, 13, 15, 26, 28, 34, 46, 51, 63, 83, 112, 114, 125, 148, 175, 177, 190, 230, 242, 253, 259, 264, 278, 280, 293.

Links

Table of n, a(n) for n=1..54.

Example

a(1) = 9: A001358(2) = 6, A001358(3) = 9, A001358(4) = 10;
a(2) = 25: A001358(8) = 22, A001358(9) = 25, A001358(10) = 26.

Mathematica

s = Select[Range[2300], Plus @@ Last /@ FactorInteger[#] == 2 &]; Transpose[ Select[ Partition[s, 3, 1], Mod[#, 2] == {0, 1, 0} &]][[2]] (* Giovanni Resta, Jun 10 2018 *)
Module[{nn=2500, sp}, sp=Select[Range[nn], PrimeOmega[#]==2&]; Select[Partition[ sp, 3, 1], Mod[ #, 2] =={0, 1, 0}&]][[;; , 2]] (* Harvey P. Dale, Mar 03 2023 *)

Crossrefs

Cf. A001358.

Sequence in context: A326742 A246328 A065772 * A090333 A137190 A044070

Adjacent sequences: A305752 A305753 A305754 * A305756 A305757 A305758

Keyword

nonn

Author

Zak Seidov, Jun 10 2018

Status

approved