Fermat tæl
In rīmcræftum, Fermat tæl, genemnod æfter Pierre de Fermat, þǣm þe hīe ærest hogde, is positif tæl mid scape: F n = 2 2 n + 1 {\displaystyle F_{n}=2^{2^{n}}+1} 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.