;+
; NAME:
;  trimultmat
; PURPOSE: (one line)
;  multiply a set of tri-diagonal matrices
; DESCRIPTION:
;  multiply a set of tri-diagonal matrices
; CATEGORY:
;  Volatile Transport
; CALLING SEQUENCE:
;   res = trimultmat(a, b, c, u)
; INPUTS:
;  a - matrix of lower sub-diagonals (a[m,1..n-1])
;  b - matrix of diagonal (b[m,0..n-1])
;  c - matrix of upper sub-diagonal (c[m,0..n-2)
;  u - matrix of vector to be multiplied
; OUTPUTS:
;  r, s.t t[i,*]= [A] # r[i,*], where A is the tridiagonal matrix
;     made of a[i,1:n-1], b[i,*], c[i,0:n-2]
; PROCEDURE:
;  
; MODIFICATION HISTORY:
;  Written 2011 Aug 27, by Leslie Young, SwRI
;  2016 Apr 11. Move into layoung/math IDL library
;-
function trimultmat, a, b, c, u
    nxnyn,u,nx,n
    r = b*u
    r[*,1:n-1] = r[*,1:n-1] + a[*,1:n-1]*u[*,0:n-2]
    r[*,0:n-2] = r[*,0:n-2] + c[*,0:n-2]*u[*,1:n-1]
    return, r
end