In MATLAB, we can produce a tridiagonal matrix of order 2*m+1.
matlab: m = 5;
matlab: diag(-m:m) + diag(ones(2*m,1),1) + diag(ones(2*m,1),-1)
In SAGE, do I have to write a function, two for-loops, or is there a convenient way? Perhaps it's already defined in one package, scipy/numpy, maxima, ....
Thanks in advance!
define a my_function, here are two example codes
The diagonal matrix is an easy one:
sage: diagonal_matrix(range(-2,3))
[-2 0 0 0 0]
[ 0 -1 0 0 0]
[ 0 0 0 0 0]
[ 0 0 0 1 0]
[ 0 0 0 0 2]
The upper and lower diagonals are trickier. The best I could come up with is:
sage: block_matrix([[ 0, identity_matrix(4) ], [ zero_matrix(1,1), 0 ]])
[0|1 0 0 0]
[0|0 1 0 0]
[0|0 0 1 0]
[0|0 0 0 1]
[-+-------]
[0|0 0 0 0]
sage: block_matrix([[ 0, zero_matrix(1,1) ], [ identity_matrix(4), 0 ]])
[0 0 0 0|0]
[-------+-]
[1 0 0 0|0]
[0 1 0 0|0]
[0 0 1 0|0]
[0 0 0 1|0]
sage: v = range(-2,6) # a list, arbitrarily given
sage: m = 3 # the off-diagonal index
sage: T = block_matrix([[zero_matrix(len(v),m), diagonal_matrix(v)],
[zero_matrix(m,m),0]])
sage: # len(v), get the length of the list to determine the dimensional of T
sage: T # result
[ 0 0 0|-2 0 0 0 0 0 0 0]
[ 0 0 0| 0 -1 0 0 0 0 0 0]
[ 0 0 0| 0 0 0 0 0 0 0 0]
[ 0 0 0| 0 0 0 1 0 0 0 0]
[ 0 0 0| 0 0 0 0 2 0 0 0]
[ 0 0 0| 0 0 0 0 0 3 0 0]
[ 0 0 0| 0 0 0 0 0 0 4 0]
[ 0 0 0| 0 0 0 0 0 0 0 5]
[--------+-----------------------]
[ 0 0 0| 0 0 0 0 0 0 0 0]
[ 0 0 0| 0 0 0 0 0 0 0 0]
[ 0 0 0| 0 0 0 0 0 0 0 0]
This is inspired by @Luca 's answer.
Asked: 2013-11-12 13:29:42 +0100
Seen: 2,072 times
Last updated: Nov 13 '13
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I don't think we have this built-in, but it would be a useful enhancement... I've opened http://trac.sagemath.org/ticket/15406 for the diag of the sort mentioned with offset.