Thâbit ibn Kurrah Number

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.

REFERENCES:

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."