A number of mechanisms have been proposed to explain this effect.

A number of different scenarios were proposed at various times.

A number of theories on the matter have been proposed.

A large number of origins have been more or less seriously proposed.

In the literature, a number of different approaches have been proposed based on this idea.

A number of national policy changes have been proposed and carried out since then.

A wide number of applications have been proposed since then.

A number of different mutual credit systems have been proposed.

A number of interim measures have been proposed, for example.

To achieve that end, a number of measures are proposed.