`contract()` and related functions in the `stokes` package

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Given a $k$-form $\phi\colon V^k\longrightarrow\mathbb{R}$ and a vector $\mathbf{v}\in V$, the contraction $\phi_\mathbf{v}$ of $\phi$ and $\mathbf{v}$ is a $k-1$-form with

[ \phi_\mathbf{v}\left(\mathbf{v}^1,\ldots,\mathbf{v}^{k-1}\right) = \phi\left(\mathbf{v},\mathbf{v}^1,\ldots,\mathbf{v}^{k-1}\right) ]

provided $k>1$; if $k=1$ we specify $\phi_\mathbf{v}=\phi(\mathbf{v})$. Function contract_elementary() is a low-level helper function that translates elementary $k$-forms with coefficient 1 (in the form of an integer vector corresponding to one row of an index matrix) into its contraction with $\mathbf{v}$; function contract() is the user-friendly front end.

We will start with some simple examples. I will use phi and $\phi$ to represent the same object.

(phi <- as.kform(1:5))

Thus $k=5$ and we have $\phi=dx^1\wedge dx^2\wedge dx^3\wedge dx^4\wedge dx^5$. We have that $\phi$ is a linear alternating map with

$$\phi\left(\begin{bmatrix}1\0\0\0\0\end{bmatrix}, \begin{bmatrix}0\1\0\0\0\end{bmatrix}, \begin{bmatrix}0\0\1\0\0\end{bmatrix}, \begin{bmatrix}0\0\0\1\0\end{bmatrix}, \begin{bmatrix}0\0\0\0\1\end{bmatrix} \right)=1$$.

The contraction of $\phi$ with any vector $\mathbf{v}$ is thus a $4$-form mapping $V^4$ to the reals with $\phi_\mathbf{v}\left(\mathbf{v}^1,\mathbf{v}^2,\mathbf{v}^3,\mathbf{v}^4\right)=\phi\left(\mathbf{v},\mathbf{v}^1,\mathbf{v}^2,\mathbf{v}^3,\mathbf{v}^4\right)$. Taking the simplest case first, if $\mathbf{v}=(1,0,0,0,0)$ then

v <- c(1,0,0,0,0)

that is, a linear alternating map from $V^4$ to the reals with

$$\phi_\mathbf{v}\left( \begin{bmatrix}0\1\0\0\0\end{bmatrix}, \begin{bmatrix}0\0\1\0\0\end{bmatrix}, \begin{bmatrix}0\0\0\1\0\end{bmatrix}, \begin{bmatrix}0\0\0\0\1\end{bmatrix}\right)=1$$.

(the contraction has the first argument of $\phi$ understood to be $\mathbf{v}=(1,0,0,0,0)$). Now consider $\mathbf{w}=(0,1,0,0,0)$:

w <- c(0,1,0,0,0)

$$\phi_\mathbf{w}\left( \begin{bmatrix}0\0\1\0\0\end{bmatrix}, \begin{bmatrix}1\0\0\0\0\end{bmatrix}, \begin{bmatrix}0\0\0\1\0\end{bmatrix}, \begin{bmatrix}0\0\0\0\1\end{bmatrix}\right)=1 \qquad\mbox{or}\qquad \phi_\mathbf{w}\left( \begin{bmatrix}1\0\0\0\0\end{bmatrix}, \begin{bmatrix}0\0\1\0\0\end{bmatrix}, \begin{bmatrix}0\0\0\1\0\end{bmatrix}, \begin{bmatrix}0\0\0\0\1\end{bmatrix}\right)=-1$$.

Contraction is linear, so we may use more complicated vectors:


We can check numerically that the contraction is linear in its vector argument: $\phi_{a\mathbf{v}+b\mathbf{w}}= a\phi_\mathbf{v}+b\phi_\mathbf{w}$.

a <- 1.23
b <- -0.435
v <- 1:5
w <- c(-3, 2.2, 1.1, 2.1, 1.8)

contract(phi,a*v + b*w) == a*contract(phi,v) + b*contract(phi,w)

We also have linearity in the alternating form: $(a\phi+b\psi)\mathbf{v}=a\phi\mathbf{v} + b\psi_\mathbf{v}$.

(phi <- rform(2,5))
(psi <- rform(2,5))
a <- 7
b <- 13
v <- 1:7
contract(a*phi + b*psi,v) == a*contract(phi,v) + b*contract(psi,v)

Contraction of products

Weintraub gives us the following theorem, for any $k$-form $\phi$ and $l$-form $\psi$:

[ \left(\phi\wedge\psi\right)\mathbf{v} = \phi\mathbf{v}\psi + (-1)^k\phi\wedge\psi_\mathbf{v}.]

We can verify this numerically with $k=4,l=5$:

phi <- rform(terms=5,k=3,n=9)
psi <- rform(terms=9,k=4,n=9)
v <- sample(1:100,9)
contract(phi^psi,v) ==  contract(phi,v) ^ psi - phi ^ contract(psi,v)

The theorem is verified. We note in passing that the object itself is quite complicated:


We may also switch $\phi$ and $\psi$, remembering to change the sign:

contract(psi^phi,v) ==  contract(psi,v) ^ phi + psi ^ contract(phi,v)

Repeated contraction

It is of course possible to contract a contraction. If $\phi$ is a $k$-form, then $\left(\phi_\mathbf{v}\right)_\mathbf{w}$ is a $k-2$ form with

$$ \left(\phi_\mathbf{u}\right)_\mathbf{v}\left(\mathbf{w}^1,\ldots,\mathbf{w}^{k-2}\right)=\phi\left(\mathbf{u},\mathbf{v},\mathbf{w}^1,\ldots,\mathbf{w}^{k-2}\right) $$

And this is straightforward to realise in the package:

(phi <- rform(2,5))
u <- c(1,3,2,4,5,4,6)
v <- c(8,6,5,3,4,3,2)

But contract() allows us to perform both contractions in one operation: if we pass a matrix $M$ to contract() then this is interpreted as repeated contraction with the columns of $M$:

M <- cbind(u,v)
contract(contract(phi,u),v) == contract(phi,M)

We can verify directly that the system works as intended. The lines below strip successively more columns from argument V and contract with them:

(o <- kform(spray(t(replicate(2, sample(9,4))), runif(2))))
V <- matrix(rnorm(36),ncol=4)
jj <- c(
   as.function(contract(o,V[,1,drop=TRUE]))(V[,-1]), # scalar
max(jj) - min(jj) # zero to numerical precision

and above we see agreement to within numerical precision. If we pass three columns to contract() the result is a $0$-form:


In the above, the result is coerced to a scalar which is returned in the form of a disord object; in order to work with a formal $0$-form (which is represented in the package as a spray with a zero-column index matrix) we can use the lost=FALSE argument:


thus returning a $0$-form. If we iteratively contract a $k$-dimensional $k$-form, we return the determinant, and this may be verified as follows:

o <- as.kform(1:5)
V <- matrix(rnorm(25),5,5)
LHS <- det(V)
RHS <- contract(o,V)

Above we see agreement to within numerical error.

Contraction from first principles

Suppose we wish to contract $\phi=dx^{i_1}\wedge\cdots\wedge dx^{i_k}$ with vector $\mathbf{v}=(v_1\mathbf{e}1,\ldots,v_k\mathbf{e}_k)$. Thus we seek $\phi\mathbf{v}$ with $\phi_\mathbf{v}\left(\mathbf{v}1,\ldots,\mathbf{v}{k-1}\right) = dx^{i_1}\wedge\cdots\wedge dx^{i_k}\left(\mathbf{v},\mathbf{v}1,\ldots\mathbf{v}{k-1}\right)$. Writing $\mathbf{v}=v_1\mathbf{e}_1+\cdots+\mathbf{e}_k$, we have

\begin{eqnarray} \phi_\mathbf{v}\left(\mathbf{v}1,\ldots,\mathbf{v}{k-1}\right) &=& dx^{i_1}\wedge\cdots\wedge dx^{i_k}\left(\mathbf{v},\mathbf{v}1,\ldots\mathbf{v}{k-1}\right)\&=& dx^{i_1}\wedge\cdots\wedge dx^{i_k}\left(v_1\mathbf{e}1+\cdots+v_k\mathbf{e}_k,\mathbf{v}_1,\ldots\mathbf{v}{k-1}\right)\&=& v_1 dx^{i_1}\wedge\cdots\wedge dx^{i_k}\left(\mathbf{e}1,\mathbf{v}_1,\ldots,\mathbf{v}{k-1}\right)+\cdots+ v_k dx^{i_1}\wedge\cdots\wedge dx^{i_k}\left(\mathbf{e}k,\mathbf{v}_1,\ldots,\mathbf{v}{k-1}\right). \end{eqnarray}

where we have exploited linearity. To evaluate this it is easiest and most efficient to express $\phi$ as $\bigwedge_{j=1}^kdx^{i_j}$ and cycle through the index $j$, and use various properties of wedge products:

\begin{eqnarray} dx^{i_1}\wedge\cdots\wedge dx^{i_k} &=& (-1)^{j-1} dx^{i_j}\wedge\left(dx^{i_1}\wedge\cdots\wedge\widehat{dx^{i_j}}\wedge\cdots\wedge dx^{i-k}\right)\ &=& (-1)^{j-1} k\operatorname{Alt}\left(dx^{i_j}\otimes\left(dx^{i_1}\wedge\cdots\wedge\widehat{dx^{i_j}}\wedge\cdots\wedge dx^{i-k}\right)\right) \end{eqnarray}

(above, a hat indicates a term's being omitted). From this, we see that $l\not\in L\longrightarrow\phi=0$ (where $L=\left\lbrace i_1,\ldots i_k\right\rbrace$ is the index set of $\phi$), for all the $dx^p$ terms kill $\mathbf{e}_l$. On the other hand, if $l\in L$ we have

\begin{eqnarray} \phi_{\mathbf{e}l}\left(\mathbf{v}_1,\ldots,\mathbf{v}{k-1}\right) &=& \left(dx^{l}\wedge\left(dx^{i_1}\wedge\cdots\wedge\widehat{dx^{l}}\wedge\cdots\wedge dx^{i_k}\right)\right)\left(\mathbf{e}l,\mathbf{v}_1,\ldots,\mathbf{v}{k-1}\right)\ &=& (-1)^{l-1}k\left(dx^{l}\otimes\left(dx^{i_1}\wedge\cdots\wedge\widehat{dx^{l}}\wedge\cdots\wedge dx^{i_k}\right)\right)\left(\mathbf{e}l,\left(\mathbf{v}_1,\ldots,\mathbf{v}{k-1}\right)\right)\ &=& (-1)^{l-1}k\left(dx^{i_1}\wedge\cdots\wedge\widehat{dx^{l}}\wedge\cdots\wedge dx^{i_k}\right)\left(\mathbf{v}1,\ldots,\mathbf{v}{k-1}\right) \end{eqnarray}

Worked example using contract_elementary()

Function contract_elementary() is a bare-bones low-level no-frills helper function that returns $\phi_\mathbf{v}$ for $\phi$ an elementary form of the form $dx^{i_1}\wedge\cdots\wedge dx^{i_k}$. Suppose we wish to contract $\phi=dx^1\wedge dx^2\wedge dx^5$ with vector $\mathbf{v}=(1,2,10,11,71)^T$.

Thus we seek $\phi_\mathbf{v}$ with $\phi_\mathbf{v}\left(\mathbf{v}_1,\mathbf{v}_2 \right)=dx^1\wedge dx^2\wedge dx^5\left(\mathbf{v},\mathbf{v}_1,\mathbf{v}_2\right)$. Writing $\mathbf{v}=v_1\mathbf{e}_1+\cdots+v_5\mathbf{e}_5$ we have

\begin{eqnarray} \phi_\mathbf{v}\left(\mathbf{v}_1,\mathbf{v}_2 \right) &=& dx^1\wedge dx^2\wedge dx^5\left(\mathbf{v},\mathbf{v}_1,\mathbf{v}_2\right)\ &=& dx^1\wedge dx^2\wedge dx^5\left(v_1\mathbf{e}_1+\cdots+v_5\mathbf{e}_5,\mathbf{v}_1,\mathbf{v}_2\right)\&=& v_1 dx^1\wedge dx^2\wedge dx^5\left(\mathbf{e}_1,\mathbf{v}_1,\mathbf{v}_2\right)+ v_2 dx^1\wedge dx^2\wedge dx^5\left(\mathbf{e}_2,\mathbf{v}_1,\mathbf{v}_2\right)\ &{}&\qquad +v_3dx^1\wedge dx^2\wedge dx^5\left(\mathbf{e}_3,\mathbf{v}_1,\mathbf{v}_2\right)+ v_4 dx^1\wedge dx^2\wedge dx^5\left(\mathbf{e}_4,\mathbf{v}_1,\mathbf{v}_2\right)\ &{}&\qquad\qquad +v_5dx^1\wedge dx^2\wedge dx^5\left(\mathbf{e}_5,\mathbf{v}_1,\mathbf{v}_2\right)\&=& v_1 dx^2\wedge dx^5\left(\mathbf{v}_1,\mathbf{v}_2\right)- v_2 dx^1\wedge dx^5\left(\mathbf{v}_1,\mathbf{v}_2\right)+0+0+ v_5 dx^1\wedge dx^2\left(\mathbf{v}_1,\mathbf{v}_2\right) \end{eqnarray}

(above, the zero terms are because the vectors $\mathbf{e}_3$ and $\mathbf{e}_4$ are killed by $dx^1\wedge dx^2\wedge dx^5$). We can see that the way to evaluate the contraction is to go through the terms of $\phi$ [that is, $dx^1$, $dx^2$, and $dx^5$] in turn, and sum the resulting expressions:

o <- c(1,2,5)
v <- c(1,2,10,11,71)
(-1)^(1+1) * as.kform(o[-1])*v[o[1]] + 
(-1)^(2+1) * as.kform(o[-2])*v[o[2]] +
(-1)^(3+1) * as.kform(o[-3])*v[o[3]]

This is performed more succinctly by contract_elementary():


The "meat" of contract()

Given a vector v, and kform object K, the meat of contract() would be

Reduce("+", Map("*", apply(index(K), 1, contract_elementary, v), elements(coeffs(K))))

I will show this in operation with simple but nontrivial arguments.

(K <- as.kform(spray(matrix(c(1,2,3,6,2,4,5,7),2,4,byrow=TRUE),1:2)))
v <- 1:7

Then the inside bit would be

apply(index(K), 1, contract_elementary, v)

Above we see a two-element list, one for each elementary term of K. We then use base R's Map() function to multiply each one by the appropriate coefficient:

Map("*", apply(index(K), 1, contract_elementary, v), elements(coeffs(K)))

And finally use Reduce() to sum the terms:

Reduce("+",Map("*", apply(index(K), 1, contract_elementary, v), elements(coeffs(K))))

However, it might be conceptually easier to use magrittr pipes to give an equivalent definition:

K                                %>%
index                              %>%
apply(1,contract_elementary,v)       %>%
Map("*", ., K %>% coeffs %>% elements) %>%

Well it might be clearer to Hadley but frankly YMMV.


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stokes documentation built on March 31, 2023, 11:58 p.m.