theorem Th3: :: MESFUN15:1

for f1 being without-infty ExtREAL_sequence
for f2 being without+infty ExtREAL_sequence holds
( Partial_Sums (f1 - f2) = (Partial_Sums f1) - (Partial_Sums f2) & Partial_Sums (f2 - f1) = (Partial_Sums f2) - (Partial_Sums f1) )