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43 | 43 | ambient |
44 | 44 | (ambient, NCPolynomialRing) |
45 | 45 | Headline |
46 | | - eported from the NCALgebra package. |
| 46 | + exported from the NCALgebra package. |
47 | 47 |
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48 | 48 | Node |
49 | 49 | Key |
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138 | 138 | L = tensorArray(s) |
139 | 139 | Description |
140 | 140 | Text |
141 | | - Returns the $k$-level component of a tensor as multi-dimensioal array, represented by a nested @TO List@. |
| 141 | + Returns the $k$-level component of a tensor as multi-dimensional array, represented by a nested @TO List@. |
142 | 142 | Example |
143 | 143 | R = QQ[t]; |
144 | 144 | X = polyPath({t,t^2}); |
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370 | 370 | pwLinPath(pwlMatrix) |
371 | 371 | Description |
372 | 372 | Text |
373 | | - Creates a piecewise lienar @TO Path@ whose increments are the columns of the given matrix. |
| 373 | + Creates a piecewise linear @TO Path@ whose increments are the columns of the given matrix. |
374 | 374 | Example |
375 | 375 | M = id_(QQ^3) |
376 | 376 | pwLinPath(M) |
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434 | 434 | wA2 = wordAlgebra(2); -- signatures of paths in dimension 2 |
435 | 435 | wA3 = wordAlgebra(3); -- signatures of paths in dimension 3 |
436 | 436 | Text |
437 | | - Then we define a path in the domain space and explicitely compute its image under the polynomial map above: |
| 437 | + Then we define a path in the domain space and explicitly compute its image under the polynomial map above: |
438 | 438 | Example |
439 | 439 | R = QQ[t]; |
440 | 440 | X = polyPath({t,t^2}) -- A path in 2 dimensional space |
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503 | 503 | Text |
504 | 504 | In this package, tensors are represented as elements of free associative algebras, using the package @TO2 {"NCAlgebra :: NCAlgebra", "NCAlgebra"}@. |
505 | 505 | More precisely, the free associative algebra on the alphabet $\{\texttt 1,...,\texttt d\}$ is isomorphic to the tensor algebra $T(\mathbb R^d)$ via the algebra homomorphism induced by $\texttt i \mapsto e_i$. This allows us to interpret tensors as non-commutative polynomials, or equivalently, linear combinations of words. |
506 | | - Given an alphabet $l$, the free assocative algebra over it can be obtained by using @TO wordAlgebra@, where the letter corresponding to $x \in l$ is represented by $\texttt{Lt}_x$. |
| 506 | + Given an alphabet $l$, the free associative algebra over it can be obtained by using @TO wordAlgebra@, where the letter corresponding to $x \in l$ is represented by $\texttt{Lt}_x$. |
507 | 507 | Example |
508 | 508 | d = 5; |
509 | 509 | l1 = {getSymbol "a", getSymbol "b", getSymbol "c"}; |
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568 | 568 | We start with the mathematical definition, based on @HREF("#ref1","[1]")@ (where the operation is called {\em right half-shuffle}). Let |
569 | 569 | $T^{\geq 1}(\mathbb{R}^d)$ be the vector space spanned by the non empty words on $d$ letters. Then the half shuffle $>>$ is defined |
570 | 570 | recursively to be $$ w >> i := wi$$ for $w$ a word and $i$ a letter and $$ w >> vi := (w >> v + v >> w)\bullet i$$ for $w, v$ words |
571 | | - and $i$ a letter, where $\bullet$ is the contatenation product on words. |
| 571 | + and $i$ a letter, where $\bullet$ is the concatenation product on words. |
572 | 572 | Text |
573 | 573 | As stated in the reference, the @TO shuffle@ on non empty words can be seen as a symmetrization of the half-shuffle. As a usage example, we verify this in a particular instance. |
574 | 574 | Example |
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623 | 623 | Description |
624 | 624 | Text |
625 | 625 | A word $l$ on the alphabet $\{1,\dots, d\}$ is a {\em Lyndon word} if it is strictly smaller, in lexicographic order, than all of its rotations. |
626 | | - To any Lyndon word we can associate an iteretaed Lie braketing $b(l)\in T(\mathbb{R}^d)$ defined iteratively as follows. If $l$ is a letter $i\in \{1,\dots, d\}$ |
| 626 | + To any Lyndon word we can associate an iterated Lie braketing $b(l)\in T(\mathbb{R}^d)$ defined iteratively as follows. If $l$ is a letter $i\in \{1,\dots, d\}$ |
627 | 627 | we simply define $$ b(i) := e_i$$ |
628 | 628 | where as ever $e_i$ is the $i-th$ vector in the canonical basis of $\mathbb{R}^d$. For the length of $l$ greater than 1 we define $$ |
629 | 629 | b(I) := [b(I_1), b(I_2)]$$ |
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