Filter coefficients, returned as a numeric vector of `n+1`

values,
where `n`

is the filter order.

`b = firls(n,f,a)`

designs a linear-phase filter
of type I (`n`

odd) and type II (`n`

)
. The output coefficients, or "taps," in `b`

obey
the relation:

b(k) = b(n+2-k), k = 1, ... , n + 1

`b = firls(n,f,a,'hilbert')`

designs a linear-phase
filter with odd symmetry (type III and type IV). The output coefficients,
or "taps," in `b`

obey the relation:

b(k)
= –b(n+2-k), k = 1, ... , n + 1

`b = firls(n,f,a,'differentiator')`

designs
type III and type IV filters, using a special weighting technique.
For nonzero amplitude bands, the integrated squared error has a weight
of (1/f)^{2}. This weighting causes the error
at low frequencies to be much smaller than at high frequencies. For
FIR differentiators, which have an amplitude characteristic proportional
to frequency, the filters minimize the relative integrated squared
error. This value is the integral of the square of the ratio of the
error to the desired amplitude.