|
| |
|
|
A101605
|
|
a(n) = 1 iff n is a product of exactly 3 primes, otherwise 0.
|
|
18
| |
|
|
0, 0, 0, 0, 0, 0, 0, 1, 0, 0, 0, 1, 0, 0, 0, 0, 0, 1, 0, 1, 0, 0, 0, 0, 0, 0, 1, 1, 0, 1, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 1, 0, 1, 1, 0, 0, 0, 0, 1, 0, 1, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 1, 0, 0, 1, 0, 1, 0, 1, 0, 0, 0, 0, 1, 1, 0, 1, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 1, 0, 0, 0, 0, 0, 1, 1, 0
(list; graph; refs; listen; history; internal format)
|
|
|
|
OFFSET
| 1,1
|
|
|
COMMENTS
| "3-almost prime numbers" are a generalization of primes and semiprimes. As explained in Weisstein: "The primes correspond to the "1-almost prime" numbers 2, 3, 5, 7, 11, ... (A000040). The 2-almost prime numbers correspond to semiprimes 4, 6, 9, 10, 14, 15, 21, 22, ... (A001358). The first few 3-almost primes are 8, 12, 18, 20, 27, 28, 30, 42, 44, 45, 50, 52, 63, 66, 68, 70, 75, 76, 78, 92, 98, 99, ... (A014612). The first few 4-almost primes are 16, 24, 36, 40, 54, 56, 60, 81, 84, 88, 90, 100, ... (A014613). The first few 5-almost primes are 32, 48, 72, 80, ... (A014614)." See A101606 for the Inverse Moebius Transform of this sequence.
|
|
|
LINKS
| Index entries for characteristic functions
Eric Weisstein's World of Mathematics, Almost Prime.
|
|
|
FORMULA
| a(n) = 1 iff n has exactly three prime factors (not necessarily distinct), else a(n) = 0. a(n) = 1 iff n is an element of A014612, else a(n) = 0.
|
|
|
EXAMPLE
| a(28) = 1 because 28 = 2 * 2 * 7 is the product of exactly 3 primes, counted with multiplicity.
|
|
|
CROSSREFS
| Cf. A014612, A101606, A001358, A014613, A014614.
Sequence in context: A115790 A025460 A169673 * A135133 A011712 A011715
Adjacent sequences: A101602 A101603 A101604 * A101606 A101607 A101608
|
|
|
KEYWORD
| easy,nonn
|
|
|
AUTHOR
| Jonathan Vos Post (jvospost3(AT)gmail.com), Dec 09 2004
|
| |
|
|
