theorem Th23: :: MATRIXJ1:23

for D being non empty set
for d being Element of D
for M1, M2 being Matrix of D
for M being Matrix of Sum (Len <*M1,M2*>), Sum (Width <*M1,M2*>),D holds
( M = block_diagonal (<*M1,M2*>,d) iff for i being Nat holds
( ( i in dom M1 implies Line (M,i) = (Line (M1,i)) ^ ((width M2) |-> d) ) & ( i in dom M2 implies Line (M,(i + (len M1))) = ((width M1) |-> d) ^ (Line (M2,i)) ) ) )