Fibonacci group

In mathematics, for a natural number ${\displaystyle n\geq 2}$, the nth Fibonacci group, denoted ${\displaystyle F(2,n)}$or sometimes ${\displaystyle F(n)}$, is defined by n generators ${\displaystyle a_{1},a_{2},\dots ,a_{n}}$and n relations:

• ${\displaystyle a_{1}a_{2}=a_{3},}$
• ${\displaystyle a_{2}a_{3}=a_{4},}$
• ${\displaystyle \dots }$
• ${\displaystyle a_{n-2}a_{n-1}=a_{n},}$
• ${\displaystyle a_{n-1}a_{n}=a_{1},}$
• ${\displaystyle a_{n}a_{1}=a_{2}}$.

These groups were introduced by John Conway in 1965.

The group ${\displaystyle F(2,n)}$is of finite order for ${\displaystyle n=2,3,4,5,7}$and infinite order for ${\displaystyle n=6}$and ${\displaystyle n\geq 8}$. The infinitude of ${\displaystyle F(2,9)}$was proved by computer in 1990.

From a group ${\displaystyle G}$and a field ${\displaystyle K}$(or more generally a ring), the group ring ${\displaystyle K[G]}$is defined as the set of all finite formal ${\displaystyle K}$-linear combinations of elements of ${\displaystyle G}$− that is, an element ${\displaystyle a}$of ${\displaystyle K[G]}$is of the form ${\displaystyle a=\sum _{g\in G}\lambda _{g}g}$, where ${\displaystyle \lambda _{g}=0}$for all but finitely many ${\displaystyle g\in G}$so that the linear combination is finite. The (size of the) support of an element ${\displaystyle a=\sum \nolimits _{g}\lambda _{g}g}$in ${\displaystyle K[G]}$, denoted ${\displaystyle |\operatorname {supp} a\,|}$, is the number of elements ${\displaystyle g\in G}$such that ${\displaystyle \lambda _{g}\neq 0}$, i.e. the number of terms in the linear combination. The ring structure of ${\displaystyle K[G]}$is the "obvious" one: the linear combinations are added "component-wise", i.e. as ${\displaystyle \sum \nolimits _{g}\lambda _{g}g+\sum \nolimits _{g}\mu _{g}g=\sum \nolimits _{g}(\lambda _{g}\!+\!\mu _{g})g}$, whose support is also finite, and multiplication is defined by ${\displaystyle \left(\sum \nolimits _{g}\lambda _{g}g\right)\!\!\left(\sum \nolimits _{h}\mu _{h}h\right)=\sum \nolimits _{g,h}\lambda _{g}\mu _{h}\,gh}$, whose support is again finite, and which can be written in the form ${\displaystyle \sum _{x\in G}\nu _{x}x}$as ${\displaystyle \sum _{x\in G}{\Bigg (}\sum _{g,h\in G \atop gh=x}\lambda _{g}\mu _{h}\!{\Bigg )}x}$.

Kaplansky's unit conjecture states that given a field ${\displaystyle K}$and a torsion-free group ${\displaystyle G}$(a group in which all non-identity elements have infinite order), the group ring ${\displaystyle K[G]}$does not contain any non-trivial units – that is, if ${\displaystyle ab=1}$in ${\displaystyle K[G]}$then ${\displaystyle a=kg}$for some ${\displaystyle k\in K}$and ${\displaystyle g\in G}$. Giles Gardam disproved this conjecture in February 2021 by providing a counterexample. He took ${\displaystyle K=\mathbb {F} _{2}}$, the finite field with two elements, and he took ${\displaystyle G}$to be the 6th Fibonacci group ${\displaystyle F(2,6)}$. The non-trivial unit ${\displaystyle \alpha \in \mathbb {F} _{2}[F(2,6)]}$he discovered has ${\displaystyle |\operatorname {supp} \alpha \,|=|\operatorname {supp} \alpha ^{-1}|=21}$.

The 6th Fibonacci group ${\displaystyle F(2,6)}$has also been variously referred to as the Hantzsche-Wendt group, the Passman group, and the Promislow group.

• An alternative proof that the Fibonacci group F(2,9) is infinite by Derek K. Holt (PostScript file).
• "Fibonacci group". Encyclopedia of Mathematics.