  The functions studied in the paper are quaternion-valued functions of a
quaternionic variable. It is show that the left slice regular functions and
right slice regular functions are related by a particular involution. The
relation between left slice regular functions, right slice regular functions
and intrinsic regular functions is revealed. The classical Laplace transform
can be naturally generalized to quaternions in two different ways, which
transform a quaternion-valued function of a real variable to a left or right
slice regular quaternion-valued function of a quaternionic variable. The usual
properties of the classical Laplace transforms are generalized to quaternionic
Laplace transforms.
