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 A082469 If n==1 or mod(n,2)==0 or mod(n,5)==0 then -1, otherwise a(n) is the smallest nonnegative integer k such that the concatenation 1(k)2(k)3(k)...(k)n is a prime. 1
 -1, -1, 1, -1, -1, -1, 2, -1, 2, -1, 35, -1, 1, -1, -1, -1, 7, -1, 147, -1, 35, -1, 49, -1, -1, -1, 10, -1, 131, -1, 71, -1, 109, -1, -1, -1, 62, -1, 43, -1, 122, -1, 103, -1, -1, -1, 7, -1, 91, -1, 74, -1, 44, -1, -1, -1, 43, -1, 88, -1, 71, -1, 182, -1, -1, -1, 20, -1, 22, -1, 839, -1, 50, -1, -1, -1, 230, -1, 206, -1, 367, -1, 472, -1, -1, -1 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,7 LINKS EXAMPLE a(9)=2 because f(9,2)=12223242526272829 is prime number and f(9,0))=10203040506070809 and f(9,1)=11213141516171819 are not. PROG (MAGMA) a:=[]; for n in [1..86] do if (n eq 1) or (n mod 2 eq 0) or (n mod 5 eq 0) then a[n]:=-1; else k:=0; while true do s:=IntegerToString(1); for i in [2..n] do s*:=IntegerToString(k)*IntegerToString(i); end for; if IsPrime(StringToInteger(s)) then a[n]:=k; break; end if; k+:=1; end while; end if; end for; a; // Jon E. Schoenfield, Nov 03 2013 CROSSREFS Sequence in context: A288196 A072883 A093101 * A206566 A088151 A286129 Adjacent sequences:  A082466 A082467 A082468 * A082470 A082471 A082472 KEYWORD sign,base AUTHOR Farideh Firoozbakht, Jun 13 2003 EXTENSIONS Missing terms added by Jon E. Schoenfield, Nov 03 2013 STATUS approved

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Last modified July 23 22:18 EDT 2019. Contains 325269 sequences. (Running on oeis4.)