Question

1. Determine in each of the following cases, whether the described system is or not a...

1. Determine in each of the following cases, whether the described system is or not a group. Explain your answers. Determine what of them is an Abelian group.
a) G = {set of integers}, a* b = a − b
b) G = {set of matrices of size 2 × 2}, A * B = A · B
c) G = {a0, a1, a2, a3, a4}, ai * aj = a|i+j|, if i+j < 5, ai *aj = a|i+j−5|, if i+j > 5, for i, j=0,1,2,3,4.
d) G = R* = {Real numbers except 0}, a ∗ b = −5ab.

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