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6.4. PROJECTION TO STETTER-STRUCTURE 101<br />

(6.52) is rewritten as:<br />

ĉ =<br />

argmin<br />

c∈C k ,||c||=1<br />

⎛<br />

W<br />

⎜<br />

⎝<br />

∣∣<br />

d 1<br />

. ..<br />

d k<br />

0<br />

.<br />

0<br />

⎞<br />

V ∗ c − v<br />

⎟<br />

⎠<br />

∣∣<br />

2<br />

2<br />

(6.53)<br />

and<br />

ĉ =<br />

argmin<br />

c∈C k ,||c||=1<br />

⎛<br />

⎜<br />

⎝<br />

∣∣<br />

d 1<br />

. ..<br />

d k<br />

0<br />

.<br />

0<br />

Which can be simplified, using ˜c = V ∗ c and ṽ = W ∗ v, <strong>to</strong>:<br />

argmin<br />

˜c∈C k ,||˜c||=1<br />

⎞<br />

2<br />

V ∗ c − W ∗ v<br />

. (6.54)<br />

⎟<br />

⎠<br />

∣∣<br />

⎛<br />

⎞<br />

2<br />

d 1 ⎜<br />

⎝<br />

. ..<br />

⎟<br />

⎠ ˜c − ṽ<br />

. (6.55)<br />

∣∣<br />

d ∣∣<br />

k<br />

Note that the size of the vec<strong>to</strong>rs ˜c and ṽ is decreased such that the dimensions match <strong>to</strong><br />

the matrix diag((d 1 ,...,d k ) T ). Note furthermore that ||c|| = ||˜c||. Now the problem<br />

can be written as:<br />

⎧<br />

k∑<br />

˜c opt = argmin f(˜c) where f(˜c) = (d i˜c i − ṽ i ) 2<br />

⎪⎨<br />

⎪⎩<br />

(6.56)<br />

subject <strong>to</strong><br />

g(˜c) =˜c 2 1 + ...+˜c 2 k − 1=0<br />

Now the theory of Lagrange multipliers can be applied because we are searching<br />

for the minimum ˜c opt of a multivariate function f(˜c) with continuous first derivatives<br />

subject <strong>to</strong> one constraint g(˜c). The theory of Lagrange multipliers is well-posed<br />

when the objective function and constraints are real-valued functions. In our case we<br />

are dealing with complex functions f(˜c) and g(˜c). Variational calculus can be easily<br />

extended <strong>to</strong> cope with this complication (see [52] and [93] for details). To prevent<br />

for complicated notation we will below present shortly how the Lagrange multiplier<br />

methods works for real-valued functions f(˜c) and g(˜c).<br />

i=1<br />

2<br />

2

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