NLP ETH - Know your semirings
In the ETH NLP course, a variety of semirings is used to compute different things (normalizer, log-normalizer, most probable path, etc.) in algorithms like viterbi for conditional random fields or CYK
In the ETH NLP course, a variety of semirings is used to compute different things (normalizer, log-normalizer, most probable path, etc.) in algorithms like viterbi for conditional random fields or CYK
Kartei Details
Karten | 10 |
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Sprache | Deutsch |
Kategorie | Informatik |
Stufe | Universität |
Erstellt / Aktualisiert | 15.02.2021 / 24.01.2022 |
Lizenzierung | Keine Angabe |
Weblink |
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Describe the Boolean Semiring
The Boolean Semiring can be used for things like recognition algorithms (e.g. can this string be expressed by a syntax tree for CYK).
\(\langle \{0,1\}, \lor, \land, 0, 1\rangle\)
Describe the Viterbi Semiring
The Viterbi Semiring corresponds to finding the most probable sequence of states. It is defined as follows:
\(\langle [0,1], \max, \times, 0, 1\rangle\)
Describe the Inside Semiring
The Inside Semiring is used to find the Normalizer of a CRF
\(\langle \mathbb{R}^+ \cup \{+\infty\}, +, \times, 0, 1\rangle\)
Describe the Real Semiring
The Real Semiring is used for shortest path if there are negative weights
\(\langle \mathbb{R} \cup \{+\infty\}, min, +, +\infty ,0\rangle\)
Describe the Tropical Semiring
The tropical semiring is used to find the shortest path in cases when there are no negative weights in the graph
\(\langle \mathbb{R}^+ \cup \{+\infty\}, min, +, +\infty ,0\rangle\)
Describe the Counting Semiring
Used to find the number of paths in a graph
\(\langle \mathbb{N}, +, \times, 0 ,1\rangle\)
Describe the Log-Semiring
The Log-Semiring can be used to compute the Log-Normalizer for CRFs or CYK
\(\langle \mathbb{R} \cup \{-\infty, +\infty\}, log(e^a + e^b), a + b, -\infty ,0\rangle\)
Note that the base can be something different than e but not = 1
How do you show that something is a semiring?
- Show that \(\langle set, \oplus,\overline{0}\rangle\) is a commutative monoid
- Show that it is closed regarding \(\oplus\)
- (a + b) + c = a + (b + c)
- 0 + a = a + 0 = a
- a + b = b + a
- Show that \(\langle set, \otimes,\overline{1}\rangle\)is a monoid
- Show that it is closed regarding \(\otimes\)
- (a x b) x c = a x (b x c)
- 1 x a = a x 1 = a
- Show that \(\overline{0}\) annihilates \(\otimes\)
- 0 x a = a x 0 = 0
- Show that \(\otimes\) distributes over \(\oplus\)
- a x (b + c) = (a x b) + (a x c)
- (a + b) x c = (a x c) + (b x c)