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 A213883 Least number k such that (10^k-j)*10^n-1 is prime for some single-digit j or 0 if no such prime with 1<=k, 0<=j<=9 exists. 4
 1, 1, 1, 1, 1, 1, 1, 1, 2, 1, 3, 2, 1, 1, 3, 5, 5, 3, 1, 3, 3, 1, 1, 9, 1, 1, 1, 1, 1, 7, 3, 6, 4, 1, 4, 4, 1, 15, 10, 1, 7, 3, 1, 3, 2, 2, 4, 6, 1, 3, 5, 20, 1, 1, 1, 8, 10, 7, 15, 10, 1, 4, 2, 5, 8, 3, 23, 11, 2, 2, 9, 3, 1, 5, 4, 1, 6, 3, 18, 2 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,9 COMMENTS j cannot be 0, 3, 6 or 9 because we are searching for repdigit primes with k-1 times the digit 9, one digit (9-j), and n least-significant digits 9 (so n+k-1 times the digit 9 in total). If j is a multiple of 3, that number is also a multiple of 3 and not prime. Conjecture: there is always at least one (k,j) solution for each n. LINKS Pierre CAMI, Table of n, a(n) for n = 1..2200 EXAMPLE Refers to the primes 89, 599, 8999, 79999, 799999, 4999999, 89999999,... MAPLE A213883 := proc(n)     for k from 1 to 2*n-1 do         for j from 0 to 9 do             if isprime( (10^k-j)*10^n-1) then                 return k;             end if;         end do:     end do:     return 0 ; end proc: # R. J. Mathar, Jul 20 2012 PROG SCRIPT DIM nn, 0 DIM jj DIM kk DIMS tt OPENFILEOUT myfile, a(n).txt LABEL loopn SET nn, nn+1 IF nn>2200 THEN END SET kk, 0 LABEL loopk SET kk, kk+1 IF kk>2*nn THEN GOTO loopn SET jj, 0 LABEL loopj SET jj, jj+1 IF jj%3==0 THEN SET jj, jj+1 IF jj>9 THEN GOTO loopk SETS tt, %d, %d, %d\,; nn; kk; jj PRP (10^kk-jj)*10^nn-1, tt IF ISPRP THEN GOTO a IF ISPRIME THEN GOTO a GOTO loopj LABEL a WRITE myfile, tt GOTO loopn CROSSREFS Cf. A213790, A213884 (corresponding j). Sequence in context: A230517 A165003 A165011 * A192190 A272729 A064034 Adjacent sequences:  A213880 A213881 A213882 * A213884 A213885 A213886 KEYWORD nonn AUTHOR Pierre CAMI, Jun 26 2012 STATUS approved

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Last modified February 23 15:57 EST 2020. Contains 332170 sequences. (Running on oeis4.)