1 | initial version |
The wanted knot is the $5_2$-knot, alias $3$-twist knot.
The sage implementation goes through braid groups. The code for it is...
sage: kft = Knots().from_table
sage: ??kft
and the active code (after the doc string) is:
from sage.groups.braid import BraidGroup
if (n, k) in small_knots_table:
m, word = small_knots_table[(n, k)]
G = BraidGroup(m)
return Knot(G(word))
else:
raise ValueError('not found in the knot table')
We can reproduce the situation as follows, without being inside the directory of /usr/lib/python3.10/site-packages/sage/knots/knot.py
from sage.knots.knot_table import small_knots_table as skt
print(skt[ (5, 2) ])
We get the result (3, [-1, -1, -1, -2, 1, -2])
. Here, sage considers $m=3$, works in the braid group $G=$BraidGroup(3)
, and builds the word [-1, -1, -1, -2, 1, -2]
, then takes the $G$-element given by this word:
sage: G.<s,t> = BraidGroup(3)
sage: m, word = skt[ (5, 2)]
sage: word
[-1, -1, -1, -2, 1, -2]
sage: G(word)
s^-3*t^-1*s*t^-1
Then it associates the knot for this braid group element. It is:
The image is linked from Braid_Representatives, search on the page for...
Already for the knot 5_2 the minimum braid is shorter than the braid produced by Vogel's algorithm. Indeed, the minimum braid is...
(the one in the picture).
The images starts by applying $s^{-3}$ - the first two strands are twisted three times that way, than the last two are twisted via $t^{-1}$, then $s$ follows, then $t^{-1}$ again. The associated braid group word has minimal length $6$, so we see six crossings in the picture. The representation sage gives is slightly different, but equivalent.
Note: The doc string of the Knots().from_table
function also gives an example with "different $n$" in the call as $n$-parameter, and in the offered knot as number of crossings:
EXAMPLES:
sage: K1 = Knots().from_table(6,3); K1
Knot represented by 6 crossings
sage: K1.alexander_polynomial()
t^-2 - 3*t^-1 + 5 - 3*t + t^2
sage: K2 = Knots().from_table(8,4); K2
Knot represented by 9 crossings
sage: K2.determinant()
19
sage: K2.signature()
2
The second knot is called for $(8,4)$, but comes with nine crossings in the chosen presentation.