|
|
A229630
|
|
a(n) is the smallest semiprime m such that 2*m^k-1 is prime for k = 1, 2, ..., n.
|
|
0
|
|
|
|
OFFSET
|
1,1
|
|
LINKS
|
|
|
EXAMPLE
|
a(5)=6 because 2*6^k-1 is prime for k=1,2,3,4,5 and 6 is the smallest semiprime with this property. Also 6 is the smallest such number.
|
|
MATHEMATICA
|
a[n_]:=(For[m=1, !(2<Length[Divisors[m]]<5&&Union[Table[PrimeQ[2m^k-1], {k, n}]]=={True}), m++]; m); Do[Print[a[n]], {n, 7}]
|
|
PROG
|
(PARI) \\ Code to find a(8), can be modified to find other terms easily
issemi(n)=bigomega(n)==2
is8(m)=for(i=2, 8, if(!ispseudoprime(2*m^i-1), return(0))); 1
|
|
CROSSREFS
|
|
|
KEYWORD
|
nonn,more
|
|
AUTHOR
|
|
|
EXTENSIONS
|
|
|
STATUS
|
approved
|
|
|
|