Lehmer sequence

If a and b are complex numbers with

${\displaystyle a+b={\sqrt {R}}}$

${\displaystyle ab=Q}$

under the following conditions:

• Q and R are relatively prime nonzero integers
• ${\displaystyle a/b}$ is not a root of unity.

Then, the corresponding Lehmer numbers are:

${\displaystyle U_{n}({\sqrt {R}},Q)={\frac {a^{n}-b^{n}}{a-b}}}$

for n odd, and

${\displaystyle U_{n}({\sqrt {R}},Q)={\frac {a^{n}-b^{n}}{a^{2}-b^{2}}}}$

for n even.

Their companion numbers are:

${\displaystyle V_{n}({\sqrt {R}},Q)={\frac {a^{n}+b^{n}}{a+b}}}$

for n odd and

${\displaystyle V_{n}({\sqrt {R}},Q)=a^{n}+b^{n}}$

for n even.

Lehmer numbers form a linear recurrence relation with

${\displaystyle U_{n}=(R-2Q)U_{n-2}-Q^{2}U_{n-4}=(a^{2}+b^{2})U_{n-2}-a^{2}b^{2}U_{n-4}}$

with initial values ${\displaystyle U_{0}=0,\,U_{1}=1,\,U_{2}=1,\,U_{3}=R-Q=a^{2}+ab+b^{2}}$. Similarly the companion sequence satisfies

${\displaystyle V_{n}=(R-2Q)V_{n-2}-Q^{2}V_{n-4}=(a^{2}+b^{2})V_{n-2}-a^{2}b^{2}V_{n-4}}$

with initial values ${\displaystyle V_{0}=2,\,V_{1}=1,\,V_{2}=R-2Q=a^{2}+b^{2},\,V_{3}=R-3Q=a^{2}-ab+b^{2}.}$