Fermat tæl

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

$F_n=2^{2^n}+1$

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 ≡ (2^a)^b + 1 ≡ (−1)^b + 1 ≡ 0 (mod 2^a + 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

$F_n=(F_{n-1}-1{)}^2+1$

$F_n=F_{n-1}+2^{2^{n-1}}{F}_0\cdots F_{n-2}$

$F_n=F_{n-1}^2-2(F_{n-2}-1{)}^2$

$F_n=F_0\cdots F_{n-1}+2$

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 Bysen:Webarchive
• Unification of Mersenne ge Fermat Numbers Bysen:Webarchive
• Prime Factors of Fermat Numbers Bysen:Webarchive

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.)