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 A213885 Number of prime numbers of the form (10^k-1)*10^n-1 with 0
 1, 0, 3, 1, 3, 2, 2, 2, 5, 1, 2, 2, 0, 4, 1, 2, 2, 2, 5, 0, 7, 2, 2, 3, 2, 4, 0, 2, 3, 2, 2, 5, 2, 2, 4, 0, 2, 3, 0, 2, 0, 1, 4, 1, 3, 1, 4, 2, 1, 4, 5, 1, 4, 2, 0, 1, 4, 3, 3, 3, 5, 1, 1, 3, 1, 3, 1, 2, 5, 4, 2, 1, 2, 3, 1, 4, 2, 2, 1, 1, 5, 2, 2, 2, 0, 2, 2, 1, 3, 4, 3, 5, 4, 1, 1, 2, 3, 2, 4, 1 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,3 COMMENTS The condition k<=2*n sets a limit of the numbers to be checked for primeness. The sequence counts a subset of the primes considered in A213886. Statistics over n=1 to 1000: 105 values of a(n) are zero, the average value of a(n) is 2.33. LINKS Pierre CAMI, Table of n, a(n) for n = 1..1000 EXAMPLE (10^1-1)*10-1=89 prime (10^2-1)*10^1-1=989 composite so a(1)=1 (10^1-1)*10^2-1=899 composite (10^2-1)*10^2-1=9899 composite (10^3-1)*10^2-1=99899 composite (10^4-1)*10^2-1=999899 composite so a(2)=0 MAPLE A213885 := proc(n)     local a, k;     a := 0 ;     for k from 1 to 2*n do         if isprime((10^k-1)*10^n-1) then             a := a+1 ;         end if;     end do:     return a; end proc: # R. J. Mathar, Jul 19 2012 PROG (PFGW64 and SCRIPTIFY) SCRIPT DIM nn, 0 DIM kk DIMS tt OPENFILEOUT myfile, a(n).txt LABEL loopn SET nn, nn+1 IF nn>1000 THEN END SET kk, 0 LABEL loopk SET kk, kk+1 IF kk>2*nn THEN GOTO loopn SETS tt, %d, %d\,; nn; kk PRP (10^kk-1)*10^nn-1, tt IF ISPRP THEN GOTO a IF ISPRIME THEN GOTO a GOTO loopk LABEL a WRITE myfile, tt GOTO loopk CROSSREFS Sequence in context: A133571 A326420 A171899 * A347739 A083208 A126682 Adjacent sequences:  A213882 A213883 A213884 * A213886 A213887 A213888 KEYWORD nonn AUTHOR Pierre CAMI, Jul 10 2012 STATUS approved

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Last modified September 19 08:05 EDT 2021. Contains 347556 sequences. (Running on oeis4.)