Chemistry - Almost prime

In mathematics, a natural number n is called k-almost prime iff Ω(n) = k, where Ω(n) is the sum of the exponents in the prime decomposition of n:

$\Omega(n) := \sum a_i \qquad\mbox{if}\qquad n = \prod p_i^{a_i}.$

A natural number is thus prime iff it is 1-almost prime, and semiprime iff it is 2-almost prime. The set of k-almost prime numbers is usually denoted by P_k. The first few k-almost prime numbers are:

k	k-almost prime numbers	OEIS sequence
1	2, 3, 5, 7, 11, 13, 17, 19, ...	A000040
2	4, 6, 9, 10, 14, 15, 21, 22, ...	A001358
3	8, 12, 18, 20, 27, 28, 30, ...	A014612
4	16, 24, 36, 40, 54, 56, 60, ...	A014613
5	32, 48, 72, 80, 108, 112, ...	A014614
6	64, 96, 144, 160, 216, 224, ...	A046306
7	128, 192, 288, 320, 432, 448, ...	A046308
8	256, 384, 576, 640, 864, 896, ...	A046310
9	512, 768, 1152, 1280, 1728, ...	A046312
10	1024, 1536, 2304, 2560, ...	A046314
11	2048, 3072, 4608, 5120, ...	A069272
12	4096, 6144, 9216, 10240, ...	A069273
13	8192, 12288, 18432, 20480, ...	A069274
14	16384, 24576, 36864, 40960, ...	A069275
15	32768, 49152, 73728, 81920, ...	A069276
16	65536, 98304, 147456, ...	A069277
17	131072, 196608, 294912, ...	A069278
18	262144, 393216, 589824, ...	A069279
19	524288, 786432, 1179648, ...	A069280
20	1048576, 1572864, 2359296, ...	A069281

External links

MathWorld: Almost prime

Categories: Number theory