|
Search: id:A130139
|
|
|
| A130139 |
|
Let f denote the map that replaces k by the concatenation of its proper divisors, written in increasing order, each divisor being written in base 10 in the normal way. Then a(n) = prime reached when starting at 2n+1 and iterating f. |
|
+0
6
|
|
| 1, 3, 5, 7, 3, 11, 13, 1129, 17, 19, 37, 23, 5, 313, 29, 31, 311, 1129, 37, 313, 41, 43
(list; graph; listen)
|
|
|
OFFSET
|
0,2
|
|
|
COMMENT
|
If 2n+1 is 1 or a prime, set a(n) = 2n+1. If no prime is ever reached, set a(n) = -1.
|
|
EXAMPLE
|
n = 7: 2n+1 = 15 = 3*5 -> 35 = 5*7 -> 57 = 3*19 -> 319 = 11*29 -> 1129, prime, so a(7) = 1129.
|
|
CROSSREFS
|
Cf. A130140, A130141, A130142. A bisection of A120716.
Sequence in context: A099984 A130141 A130142 this_sequence A101088 A134487 A064537
Adjacent sequences: A130136 A130137 A130138 this_sequence A130140 A130141 A130142
|
|
KEYWORD
|
base,more,nonn
|
|
AUTHOR
|
N. J. A. Sloane (njas(AT)research.att.com), Jul 30 2007
|
|
EXTENSIONS
|
The value of a(22) is currently unknown.
|
|
|
Search completed in 0.002 seconds
|
