|
| |
|
|
A217263
|
|
Composite numbers such that every concatenation of their prime factors is prime.
|
|
4
|
|
|
|
21, 33, 51, 63, 93, 111, 133, 177, 201, 219, 247, 253, 327, 411, 427, 549, 573, 589, 657, 679, 687, 763, 793, 813, 833, 889, 993, 1077, 1081, 1119, 1127, 1243, 1339, 1347, 1401, 1411, 1497, 1501, 1603, 1623, 1651, 1671, 1821, 1839, 1843, 1851, 1981, 2009, 2019
(list;
graph;
refs;
listen;
history;
text;
internal format)
|
|
|
|
OFFSET
|
1,1
|
|
|
COMMENTS
|
The smallest term with 3 or more prime factors is 63 = 3*3*7 (see A217264).
The smallest term with 4 or more prime factors is 21249 = 3*3*3*787 (see A217265).
The smallest term with 5 or more prime factors is 146461 = 7*7*7*7*61.
There is no term under 10^8 with 6 or more prime factors.
The smallest term with 3 or more distinct prime factors is 3311 = 7*11*43 (see A180679).
There is no term under 10^8 with 4 or more distinct prime factors.
|
|
|
LINKS
|
Table of n, a(n) for n=1..49.
|
|
|
EXAMPLE
|
21 is 3*7. Both 37 and 73 are prime, so 21 is in the sequence.
63 is 3*3*7. 337, 373 and 733 are all prime, so 63 is in the sequence.
|
|
|
CROSSREFS
|
Cf. A217264, A217265, A180679, A181559. Related sequences: A105184, A019549 and A106582.
Sequence in context: A339963 A141249 A026068 * A330441 A176945 A333857
Adjacent sequences: A217260 A217261 A217262 * A217264 A217265 A217266
|
|
|
KEYWORD
|
nonn,base
|
|
|
AUTHOR
|
Dmitri Kamenetsky, Sep 29 2012
|
|
|
STATUS
|
approved
|
| |
|
|
