The OEIS Foundation is supported by donations from users of the OEIS and by a grant from the Simons Foundation.

 Hints (Greetings from The On-Line Encyclopedia of Integer Sequences!)
 A246718 a(n) is the number of different ways of concatenating the numbers {3^k, k=0,...,n} so as to produce a prime number. 0
 2, 2, 3, 44, 128, 619, 4134, 28628, 229132, 2107538, 21438790, 238754555 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS A PARI program distinct from that below was used to compute a(14) using four windows in under a month, but the value was lost. It is neither trivial nor very difficult to establish that distinct permutations lead to distinct values. LINKS EXAMPLE The a(1)+a(2)+a(3)+a(4)=51 primes corresponding to the first four terms are, in increasing order, 13, 31, 139, 193, 12739, 19273, 32719, 1273981, 1278139, 1279813, 1381279, 1398127, 1812793, 1819273, 1927813, 2713981, 2718139, 2718193, 2731819, 2738119, 2738191, 2739181, 2781139, 2781193, 2781913, 2793181, 2793811, 2798113, 3127819, 3127981, 3192781, 3271981, 3279811, 3811279, 3812719, 3812791, 3912781, 3918127, 8113279, 8113927, 8119273, 8127319, 8131927, 8139127, 8193127, 9127813, 9181327, 9273181, 9327181, 9812731 and 9813127. Concatenations not shown, such as 931 = 7^2 * 19 and 1392781 = 13 * 107137, are all composite. PROG (PARI) a(n, v=vector(n+1, k, Str(3^(k-1))))=sum(k=1, (n+1)!, ispseudoprime(eval(concat(vecextract(v, numtoperm(n+1, k)))))) \\ M. F. Hasler, Jan 13 2015 CROSSREFS Sequence in context: A078239 A083113 A184847 * A177764 A027498 A094877 Adjacent sequences:  A246715 A246716 A246717 * A246719 A246720 A246721 KEYWORD base,nonn AUTHOR James G. Merickel, Nov 15 2014 EXTENSIONS Edited and verified up to n=9 by M. F. Hasler, Jan 13 2015 STATUS approved

Lookup | Welcome | Wiki | Register | Music | Plot 2 | Demos | Index | Browse | More | WebCam
Contribute new seq. or comment | Format | Style Sheet | Transforms | Superseeker | Recent
The OEIS Community | Maintained by The OEIS Foundation Inc.

Last modified September 29 14:39 EDT 2020. Contains 337432 sequences. (Running on oeis4.)