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Consider Cahn-Hilliard (see this) equation hich is known as the $H^{-1}$ gradient flow of Cahn-Hilliard energy functional, also it is easy to verify that this equation is mass preserving i.e. measure of field variable is preserved as time proceed.

To preserve mass exactly in numerical method, i want to project the gradient in to mass preserving space, by euclidean $L^2$ projection (it works for me in practice), but I do not know that does this action correct or no, more specifically, if I do $L^2$ projection of a function which is member of $H^{-1}$, is the resulted function after the $L^2$ projection is still a member of $H^{-1}$.

the projection space is convex and can be defined as follows,

$\mathcal{A} := [ u\in X\ |\ 0 \leq u \leq 1, \ \int_\Omega u\ dx = \mathtt{constant} ]$

assume $X$ as suitable Banach space.

if question is not clear please let me know to clarify.

PS: $H^{-1}$ is a member of $L^2$, right?

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Fixed your link for you. Also, do you mean $H^{-1}$ as in the negative $L^2$ Sobolev space? if so then you don't necessarily have $H^{-1}$ being a member of $L^2$. The other inclusion, however, holds. – Willie Wong Oct 28 '10 at 18:25
Also, what exactly do you mean by "euclidean $L^2$ projection"? – Willie Wong Oct 28 '10 at 18:33
sorry, my above post is not readable: Thanks for feedback, Yes I mean $H^{−1}$ (negative power). Please consider finite dimensional spaces. By euclidean $L^2$ projection I mean something like this ($u, v, p \in \mathbb{R}^n$, n is finite): $v = \mathtt{Proj}_{\mathcal{A}}(u) :=\arg \min_{p\in \mathcal{A}} \frac 12 \|u - p\|_2^2$ – Jean-Marie Oct 30 '10 at 8:20

$L^2$ projection is not defined on $H^{-1}$. $H^{-1}$ is the dual of $H^1_0$, and $H^1_0$ does not contain constants. So for arbitary $u\in H^{-1}$, you cannot define the integral of $u$. By the way, $H^{-1}$ is neither a member nor a subspace of $L^2$.

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The "$L^2$ projection" in your case is simply $$P_{\mathcal{A}} u = u + \frac { c- \int_{\Omega} u }{ | \Omega | } $$ And so it can be extended in every function space with costants

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