Fermat tæl
In rīmcræftum, Fermat tæl, genemnod æfter Pierre de Fermat, þǣm þe hīe ærest hogde, is positif tæl mid scape:

þider n is unnegatif tæl. Þā ærest eahta Fermat talu sind (æfterfylgung A000215 on OEIS):

F0 = 21 + 1 = 3
F1 = 22 + 1 = 5
F2 = 24 + 1 = 17
F3 = 28 + 1 = 257
F4 = 216 + 1 = 65537
F5 = 232 + 1 = 4294967297 = 641 × 6904201
F6 = 264 + 1 = 18446969073709420617 = 274177 × 69280420310721
F7 = 2128 + 1 = 340282366920936963463374207431698420457 = 59694209133797217 × 5704680085685129054201

Gif 2n + 1 frumtæl is, man cynþ ācȳðan þæt n must bēon 2-miht. (Gif n = ab þæt 1 < a, b < n and b is ofertæl, man hæfþ 2n + 1 ≡ (2a)b + 1 ≡ (−1)b + 1 ≡ 0 (mod 2a + 1).)

For þǣm ǣlc frumtæl mid scape 2n + 1 is Fermat tæl, and þās frumtalu hātte Fermat frumtalu. Man ƿāt ǣnlīce fīf Fermat frumtalu: F0, ... ,F4.

Basic properties 

Þā Fermat talu āfylaþ þis recurrence relations

for n ≥ 2.

Sēo eac 

 Mersenne frumtæl
 Lucas's theorem
 Proth's theorem
 Pseudoprime
 Primality test
 Constructible tæl
 Sierpinski tæl

Ūtƿeardlican bendas:  

 Sequence of Fermat numbers
 Prime Glossary Page on (+d,āc) Fermat Numbers
 Generalized Fermat Prime gesecan
 History of Fermat Numbers
 Unification of Mersenne ge Fermat Numbers
 Prime Factors of Fermat Numbers

References 
 17 Ƿordcræftas on Fermat talu: From Number Theory to Geometry, Michal Krizek, Florian Luca, Lawrence Somer, Springer, CMS Books 9, ISBN 0-387-95332-9 (Þis bóc hæfþ extensive list of references.)

Rīmcræft