tril
Lower triangular part of a matrix.
t=tril(a)
t=tril(a,k)
Inputs 
a 
Any matrix. 
k 
An integer. 
Outputs 
t 
The lower triangular part of a , or the 'pseudo' lower triangular part as defined below.

Description
The output is of the same size as a
. If k
is not specified, the output
is a copy of the input except that all the elements in the upper triangular
part, i.e. the elements of the type t[i,j]
for i
less than j
are replaced by zeros in
the output.
If k
is specified to be positive, all the elements above the k'th
super
diagonal are zero in in the output, i.e., the elements
of the type t[i,j],i<(jk)
are set to zero.
If k
is specified to be negative, all the elements below the (k)'th
sub
diagonal are zero in in the output, i.e., the elements
of the type t[i,j],i<(jk)
are set to zero.
Example>>a=[1 2 3 4;5 6 7 8; 9 10 11 12]
>>a
1 2 3 4
5 6 7 8
9 10 11 12
>>tril(a)
1 0 0 0
5 6 0 0
9 10 11 0
>>tril(a,1)
1 2 0 0
5 6 7 0
9 10 11 12
>>tril(a,2)
1 2 3 0
5 6 7 8
9 10 11 12
>>tril(a,1)
0 0 0 0
5 0 0 0
9 10 0 0