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 A306861 The concatenation kpk is the number obtained by placing k leading and trailing 1's around the prime p; a(n) is the smallest k such that kpk is prime, where p = prime(n), or -1 if no such k exists. 1
 -1, 1, 1, 3, -1, 3, 1, 21, 1, 1, 2, -1, 3, 2, 1, 1, 42, 14, 3, 73, 3, 2, 1, 4, 3, -1, 2, 1, 3, 1, 3, 1, 3, 3, 1, 6, 2, 3, 192, 1, 4, 3, 3, 8, 1, 9, 36, 5, 12, 5, 18, 1, 26, 1, 16, 10, 15, 2, 72, 22, 3, 4, 2, 4, 5, 1, 12, 5, 13, 3, 9, 1, 6, 60, 2, 1, 58 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,4 COMMENTS For p = 2,11,37,101 kpk is composite for all k, hence a(n) = -1. For p = 397 (n=78), 563, 739, 1249, ... no k (<= 12000) has yet been found such that kpk is prime, but also there is no proof yet that k does not exist. If p = prime(n) is an odd repunit prime, a(n) is half the difference in repunit length between p and the next repunit prime. Conjecture: There are infinitely many -1 terms in this sequence. LINKS Hans Havermann, Prime sandwiches (includes a link to a table giving particulars for all primes <10^5) EXAMPLE a(1) = -1 because k2k is divisible by the (k+1)-th repunit for all k. The same argument applies to a(26) (p=101). a(2)=1 since 131 is prime, a(3)=1 since 151 is prime, a(4)=3 since 1117111 is prime. a(5)=-1 because k11k is always divisible by 11. a(12) = -1 because the factor cycle for k37k comprises a covering congruence as follows: k==1 (mod 3)-->3|k37k; k==2 (mod 3)--> 13|k37k; k==3 (mod 3)--> 37|p37p. For a(78) (p=397) no k (up to 30000) has been found such that kpk is prime. MAPLE Wrapped_prime := proc (p::prime, N::posint := 5000) local n, k, m0, m; n := length(p); for k to N do m0 := add(10^i, i = 0 .. k-1); m := m0+10^k*p+10^(k+n)*m0; if isprime(m) then return k end if end do end proc Wrapped_prime(p). #Enter a value for p in this line and the code will calculate the first k for which kpk is prime (up to a max value of N, which can be chosen arbitrarily). CROSSREFS Cf. A002275, A004022, A069687. Sequence in context: A212183 A115716 A079412 * A262940 A278601 A281038 Adjacent sequences:  A306858 A306859 A306860 * A306862 A306863 A306864 KEYWORD sign,base,more AUTHOR David James Sycamore, Mar 14 2019 STATUS approved

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Last modified August 21 06:54 EDT 2019. Contains 326162 sequences. (Running on oeis4.)