321 number

A Thâbit ibn Kurrah number, sometimes called a 321-number, is a number of the form K_n = 3·2^n - 1. The first few for n = 0, 1, ... are 2, 5, 11, 23, 47, 95, 191, 383, 767, ... (OEIS A055010). Note that there exist infinitely many odd integers k such that k·2^n - 1 is composite for every n>=1, and numbers k with this property are called Riesel numbers.

Cunningham number | integer sequence primes | Pierpont prime | Riesel number | Thâbit ibn Kurrah prime | Thâbit ibn Kurrah rule | Woodall number