Let
p and
q be two fixed no
n zero integers verifying the condition
gcd(
p,
q) = 1. We check solutions in non
zero integers
a1,
b1,
a2,
b2 and
a3 for the following Diophantine equations: (
B1)

(
B2)

.
The equations (
B1) and (
B2) were considered by R.C. Lyndon and J.L. Ullman in [1] and A.F. Beardon in [2] in connection with the
freeness of the M
?bius group

generated by two matrices of

namely

and

where

. They proved that if one of the equations (
B1) or (
B2) has solutions in non zero integers then the group

is not free. We give algorithms to decide if
these equations admit solutions. We obtain an arithmetical criteria on
p and
q for which (B1)
admits solutions. We show that for all
p and
q the equations (
B1) and (
B2) have only a
finite number of solutions.