  A general model for opinion formation and competition, like in ideological
struggles is formulated. The underlying set is a closed one, like a country but
in which the population size is variable in time. Several ideologies compete to
increase their number of adepts. Such followers can be either converted from
one ideology to another or become followers of an ideology though being
previously ideologically-free. A reverse process is also allowed. We consider
two kinds of conversion: unitary conversion, e.g. by means of mass
communication tools, or binary conversion, e.g. by means of interactions
between people. It is found that the steady state,when it exists, depends on
the number of ideologies. Moreover when the number of ideologies increases some
tension arises between them. This tension can change in the course of time. We
propose to measure the ideology tensions through an appropriately defined scale
index.
