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 A104644 a(n) is the smallest prime divisor of the number obtained from concatenation of the first n primes. 4
 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 Dario Alejandro Alpern, Factorization using the Elliptic Curve Method. Hisanori Mishima, Smarandache consecutive prime sequences (n = 1 to 100). 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. MAPLE with(numtheory): T:=proc(t) local x, y; x:=t; y:=0; while x>0 do x:=trunc(x/10); y:=y+1; od; end: P:=proc(q) local a, b, n; b:=2; print(2); for n from 2 to q do b:=ithprime(n)+b*10^T(ithprime(n)); a:=sort([op(divisors(b))]); print(a[2]); od; end: P(10^6); # Paolo P. Lava, Apr 30 2014 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 Cf. A019158, A020639, A074809. Sequence in context: A120713 A167920 A237579 * A158992 A128365 A153654 Adjacent sequences:  A104641 A104642 A104643 * A104645 A104646 A104647 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

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Last modified May 21 23:42 EDT 2019. Contains 323472 sequences. (Running on oeis4.)