Intended for: July 8, 2012
Timetable
 First draft entered by Alonso del Arte on March 12, 2011 based on an almost verbatim copy of a writeup by Karsten Meyer from January 1, 2011. ✓
 Draft reviewed by Daniel Forgues on May 5, 2011 ✓
 Draft approved by Alonso del Arte on July 7, 2011 ✓
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A181780: Numbers
which are
Fermat pseudoprimes to some base
, with
.

{ 15, 21, 25, 28, 33, 35, 39, ... }
For example
15 is Fermat pseudoprime to every base of the form
and
with
. The sequence shows that the distribution of the Fermat pseudoprimes is the opposite of rare, if it is not determined on a fixed base
. There are
429 Fermat pseudoprimes below
1000, but only
168 prime numbers. Fermat pseudoprimes to all bases
, with
are called
Carmichael numbers.