# how to write QR algorithm into one equation to represent?

http://en.wikipedia.org/wiki/QR_algorithm

is it possible to write it in one equation

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You mean you don't want to write $A = QR$, but want a shorthand for it? – Amzoti Feb 1 '13 at 2:27
i mean Q = something, R = something, rather than an algorithm – Machine Gun Feb 1 '13 at 2:44
Interesting question, though any answer would be far more conceptual than anything. $A=QR$ gives $Q=AR^{-1}$ and $R=Q^{-1}A$, but that is about it. To find orthogonal $Q$ and triangular $R$ for $A=QR$ would require an algorithm, not a simple formula/equation. So I guess the answer is no. – adam W Feb 1 '13 at 2:53
after read your comment, it sounds right. – Machine Gun Feb 1 '13 at 3:15
still hope another way to do this algorithm – Machine Gun Feb 1 '13 at 3:24

The factors $Q$ and $R$ can indeed be written using explicit formulas, since the columns of $Q$ are given by the Gram-Schmidt process, and can thus be expressed by (increasingly complicated) functions of the entries of $A$ that are continuous away from $\det A =0$. As adam points out, once you have $Q$, you also have $R$.
The output of the $QR$ algorithm is the spectrum of $A$. It does have a formula, in a sense, given by the characteristic polynomial; an explicit formula for the eigenvalues of $A$ would also be an explicit formula for the roots of all monic polynomial. To my knowledge no such formula exists, and by the Abel-Ruffini theorem it cannot be algebraic.
That depends on the decomposition. Like I say in the first part of my answer, the $QR$ decomposition has a formula. So do the $LU$ and Cholesky decompositions. SVD on the other hand does not. – user7530 Feb 1 '13 at 4:24