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A213885 Number of prime numbers of the form (10^k-1)*10^n-1 with 0<k<=2*n 2
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.)