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A214502
The size of the set of numbers k>=0 such that all (3^n+k)*2^n-1 are prime but only the last (largest) (3^n+k)*2^n+1 is also an associated twin prime.
3
1, 3, 2, 5, 16, 2, 2, 18, 2, 8, 8, 10, 14, 9, 5, 43, 15, 1, 56, 13, 5, 151, 7, 20, 107, 3, 30, 8, 16, 31, 8, 21, 3, 103, 57, 38, 28, 37, 99, 5, 15, 50, 87, 31, 67, 107, 1, 113, 69, 12, 41, 19, 23, 43, 150, 100, 49, 76, 3, 159, 48, 86, 49, 81, 62, 48, 118, 66
OFFSET
1,2
COMMENTS
Starting at a count of zero, we consider for increasing k>=0 the pairs (3^n+k)*2^n+-1. If the smaller of these two numbers is prime, we increase the counter. If the larger of these two numbers is also prime, we admit the counter to the sequence. It is basically a measure of how many unsuccessful primality tests on the larger of the two numbers are done before it becomes a compatible twin prime.
LINKS
PROG
PFGW64 and SCRIPTIFY
SCRIPT
DIM nn, 0
DIM kk
DIM jj
DIMS tt
OPENFILEOUT myfile, b(n).txt
OPENFILEOUT myf, a(n).txt
LABEL loopn
SET nn, nn+1
SET jj, 0
IF nn>500 THEN END
SET kk, 0
LABEL loopk
SET kk, kk+1
SETS tt, %d, %d\,; nn; kk
PRP (3^nn+kk)*2^nn-1, tt
IF ISPRP THEN GOTO a
IF ISPRIME THEN GOTO a
GOTO loopk
LABEL a
SET jj, jj+1
PRP (3^nn+kk)*2^nn+1, tt
IF ISPRP THEN GOTO d
IF ISPRIME THEN GOTO d
GOTO loopk
LABEL d
WRITE myfile, tt
SETS tt, %d, %d\,; nn; jj
WRITE myf, tt
GOTO loopn
CROSSREFS
KEYWORD
nonn
AUTHOR
Pierre CAMI, Jul 20 2012
STATUS
approved