This site is supported by donations to The OEIS Foundation.

 Hints (Greetings from The On-Line Encyclopedia of Integer Sequences!)
 A130141 Let f denote the map that replaces k by the concatenation of its proper divisors, written in decreasing 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. 4
 1, 3, 5, 7, 3, 11, 13, 53, 17, 19, 73, 23, 5, 313, 29, 31, 113 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,2 COMMENTS If 2n+1 is 1 or a prime, set a(n) = 2n+1. If no prime is ever reached, set a(n) = -1. Comment from Klaus Brockhaus, Aug 01 2007: "The sequence, starting from the beginning but writing 0 when a number with more than 90 digits is reached, is: 1,3,5,7,3,11,13,53,17,19,73,23,5,313,29,31,113,0,37,197,41,43,0,47,7,173,53,0,193, 59,61,0,0,67,233,71,73,0,391393,79,9313991471335749211973,83,0,293,89,137,313, 0,97,0,101,103,0,107,109,373,113,0,391393,593,11,1993,0,127,433,131,197,0,137,139, 0,0,0,0,149,151,511793,0,157,1141931201,6113,163,0,167,13,0,173,0,593,179,181, 613,12575251553,0,0,191,193,0,197,199,673,..." Contribution from Andrew Carter (acarter09(AT)newarka.edu), Dec 16 2008: (Start) If this sequence were to do all terms, all even terms except 2 and 4 would be -1: a simple proof is that an even number's smallest proper divisor is 2, all even numbers except 2 and 4 have factors in front so the resulting number would be 10n+2 and n is an integer greater than 1, and therefore an even number endng in 2 other than two - rinse and repeat (End) LINKS EXAMPLE n = 13: 2n+1 = 27 has proper divisors 3 and 9, so we get 93, which has proper divisors 3 and 31, so we get 313, prime. So a(13) = 313. CROSSREFS Cf. A130139, A130140, A130142, A120716. Sequence in context: A199423 A100029 A099984 * A130142 A130139 A204938 Adjacent sequences:  A130138 A130139 A130140 * A130142 A130143 A130144 KEYWORD base,more,nonn AUTHOR Carsten Lund (lund(AT)research.att.com), Jul 30 2007, Aug 01 2007 EXTENSIONS The value of a(17) is currently unknown. STATUS approved

Lookup | Welcome | Wiki | Register | Music | Plot 2 | Demos | Index | Browse | More | WebCam
Contribute new seq. or comment | Format | Transforms | Puzzles | Hot | Classics
Recent Additions | More pages | Superseeker | Maintained by The OEIS Foundation Inc.

Content is available under The OEIS End-User License Agreement .

Last modified June 19 19:50 EDT 2013. Contains 226416 sequences.