login
The OEIS is supported by the many generous donors to the OEIS Foundation.

 

Logo
Hints
(Greetings from The On-Line Encyclopedia of Integer Sequences!)
A104644 a(n) is the smallest prime divisor of the number obtained from concatenation of the first n primes. 5
2, 23, 5, 2357, 7, 23, 11, 7, 61, 3, 17, 7, 25391, 6899, 1597, 3, 17, 3, 1019, 3, 17, 211, 41378209, 3, 479, 3, 262343078386137703, 3, 107, 3, 643, 3, 7, 3, 13, 3, 107069, 7, 14733071537, 3, 61911427, 3, 13, 3, 31, 3, 11, 59, 7, 647, 31, 3, 13, 3, 31517, 43, 3, 19 (list; graph; refs; listen; history; text; internal format)
OFFSET
1,1
LINKS
FORMULA
a(n) = A020639(A019518(n)). - Michel Marcus, Aug 01 2017
EXAMPLE
n=5, concatenation of {2,3,5,7,11} is 235711=7*151*223, smallest prime factor is a(5)=7.
MATHEMATICA
Map[FactorInteger[FromDigits@ #][[1, 1]] &, Rest@ FoldList[Join @@ {#1, IntegerDigits[#2]} &, {}, Prime@ Range@ 29]] (* Michael De Vlieger, Jul 31 2017 *)
PROG
(PARI) a(n) = vecmin(factor(eval(concat(concat([""], primes(n)))))[, 1]); \\ Michel Marcus, Aug 01 2017
CROSSREFS
Sequence in context: A309154 A167920 A237579 * A158992 A128365 A326954
KEYWORD
nonn,base
AUTHOR
Labos Elemer, Mar 18 2005
EXTENSIONS
a(29) and further terms from Robert G. Wilson v, Mar 18 2005
Edited by Charles R Greathouse IV, Apr 23 2010
STATUS
approved

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

License Agreements, Terms of Use, Privacy Policy. .

Last modified July 19 22:34 EDT 2024. Contains 374441 sequences. (Running on oeis4.)