# How to get upper triangluar matrix from lower triangular matrix?

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gliang on 11 Jul 2011
Answered: Veera Kanmani on 20 Apr 2018
If L'L = U'U, where L and U are lower and upper triangular matrices respectively, then how to get U directly from L?
P.S.: I already knew one solution is U = QR (L), but this operation need O(N^3) if L's size is N*N. I am wondering whether there is a O(N^2) operation for solving triangular matrix.
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### Answers (1)

Veera Kanmani on 20 Apr 2018
https://in.mathworks.com/matlabcentral/answers/152818-how-to-covert-a-vector-of-lower-triangular-matrix-factors-back-to-the-original-matrix
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