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 A083803 Smallest number k such that the concatenations k 2n-1 and 2n-1 k are both primes, or 0 if n = 5m+3. 1
 1, 1, 0, 1, 7, 3, 1, 0, 3, 7, 13, 11, 0, 1, 27, 1, 7, 0, 3, 7, 9, 39, 0, 9, 1, 19, 51, 0, 1, 3, 7, 1, 0, 3, 49, 9, 3, 0, 17, 19, 1, 9, 0, 7, 23, 1, 11, 0, 1, 1, 77, 1, 0, 33, 1, 109, 11, 0, 1, 3, 3, 1, 0, 7, 1, 51, 39, 0, 3, 21, 59, 21, 0, 13, 47, 1, 1, 0, 9, 7, 9, 19, 0, 29, 3, 37, 149, 0, 7, 21 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,5 COMMENTS Observation: Most of the members are prime or 1. The first composite k are a(15)=27, a(21)=9, a(22)=39, a(24)=9, a(27)=51, a(35)=49.... LINKS EXAMPLE a(19) = 7 as 197 and 719 both are prime, a(15) =0. MAPLE A083803 := proc(n)     if modp(n, 5) = 3 then         0;     else         for k from 1 do             if isprime(digcat2(k, 2*n-1) ) then                 if isprime(digcat2(2*n-1, k)) then                     return k;                 end if;             end if;         end do:     end if; end proc: # R. J. Mathar, Aug 23 2014 CROSSREFS Sequence in context: A253905 A154159 A334060 * A136595 A111475 A222581 Adjacent sequences:  A083800 A083801 A083802 * A083804 A083805 A083806 KEYWORD base,nonn AUTHOR Amarnath Murthy and Meenakshi Srikanth (menakan_s(AT)yahoo.com), May 08 2003 EXTENSIONS More terms from David Wasserman, Nov 30 2004 STATUS approved

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Last modified May 10 22:16 EDT 2021. Contains 343780 sequences. (Running on oeis4.)