A Thâbit ibn Kurrah number, sometimes called a 321-number, is a number of the form.
The first few for , 1, ... are 2, 5, 11, 23, 47, 95, 191, 383, 767, ... (OEIS
A055010).
Note that there exist infinitely many odd integers such that is composite for every , and numbers with this property are called Riesel
numbers.
Riesel, H. "Lucasian Criteria for the Primality of ."
Math. Comput.23, 869-875, 1969.Sloane, N. J. A.
Sequence and A055010 in "The On-Line
Encyclopedia of Integer Sequences."
.
The first few for 
, 1, ... are 2, 5, 11, 23, 47, 95, 191, 383, 767, ... (OEIS
A055010).
such that 
is composite for every 
, and numbers 
with this property are called Riesel
numbers.
."
Math. Comput. 23, 869-875, 1969.Sloane, N. J. A.
Sequence and