consisting of πΓπ rectangular arrays and
acted on the matrix algebraMβ,πβ‘(π )
so that for π΅=(πππ)π,ππ=1,π=1βMπ,πβ‘(π) and π΄=(πππ)β,ππ=1,π=1βMπ,πππππβ‘(π ) we have
πΆ=π΄π΅=(πππ)β,ππ=1,π=1