Left, Right Inverses: Multiple Left Inverses and No Right Inverse?

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SUMMARY

The discussion centers on the properties of linear transformations and matrices, specifically addressing the implications of having multiple left inverses. It is established that if a matrix A has multiple left inverses, such as B1 and B2, it cannot possess a right inverse. This conclusion arises from the contradiction that would occur if both left and right inverses existed, leading to the assertion that a matrix is invertible only when both inverses are equal to a unique inverse.

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Suppose we have a linear transformation/matrix A, which has multiple left inverses B1, B2, etc., such that, e,g,:

[tex]B_1 \cdot A = I[/tex]

Can we conclude from this (i.e., from the fact that A has multiple left inverses) that A has no right inverse?

If so, why is this?
 
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Suppose there were a right inverse, say, R. Multiplying the equation [itex]B_1A= I[/itex] on the right by R gives [itex](B_1A)R= IR[/itex] so that [itex]B_1(AR)= B_1= R[/itex] But then doing the same with [itex]B_2A[/itex] leads to [itex]B_2= R[/itex].

In other words, if a matrix has both right and left inverses, then it is invertible and both right and left inverses are equal to its (unique) inverse.
 

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