The twentieth Fermat number, F20 = 22^20 +1, has been proven composite by machine computation.
Published in Mathematics of Computation, Volume 50, Issue 181, 1988, pages 261-263.
Young, J. and Buell, D.A. (1988). The twentieth Fermat number is composite. Mathematics of Computation, 50(181), 261-263.
First published in Mathematics of Computation in 1988, published by the American Mathematical Society.
Copyright ©1988 American Mathematical Society.