connected sum of knots Anonymous

Hi! I was trying to recursively construct connected sums of knots, but I seem to run in to some problems when connect summing a knot to itself more than two times:

B = BraidGroup(2)

trefoil = Knot(B([1,1,1]))

K = trefoil.connected_sum(trefoil)

L = K.connected_sum(trefoil)

This does not work:

ValueError                                Traceback (most recent call last)

ipython-input-1-7b977cf99468 in module()

2 trefoil = Knot(B([Integer(1),Integer(1),Integer(1)]))

3 K = trefoil.connected_sum(trefoil)

--> 4 L = K.connected_sum(trefoil)

/home/sc_serv/sage/local/lib/python2.7/site-packages/sage/knots/knot.pyc in connected_sum(self, other)

294         return Knot(B(list(b1.Tietze())

295                       + [(abs(i) + b2s) * Integer(i).sign() for i in b2.Tietze()]

--> 296                       + [b1s]))

297

/home/sc_serv/sage/local/lib/python2.7/site-packages/sage/knots/knot.pyc in __init__(self, data, check)

104         if check:

105             if self.number_of_components() != 1:

--> 106                 raise ValueError("the input has more than 1 connected component")

107

108     def __repr__(self):

ValueError: the input has more than 1 connected component

Am I doing something wrong here? Thanks in advance!

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1

Note that, calling t the trefoil knot and tt its connected sum with itself, one gets different errors when trying the connected sum of tt with t, versus of t with tt. Indeed, defining:

B = BraidGroup(2)
t = Knot(B([1, 1, 1]))
c = Knot.connected_sum
tt = c(t, t)

one attempt gives:

sage: c(tt, t)
Traceback (most recent call last)
...
ValueError: the input has more than 1 connected component

while the other attempt gives:

sage: c(t, tt)
Traceback (most recent call last)
...
ValueError: generators not in the group

so it does not work either way? the result should be the same! anyway, it seems to have problems even when all knots are called in different ways, very weird!

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This points to a bug in the code to compute connected_sum.

This issue is now tracked at

Thanks for reporting!

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