Now we vary the width of the system with flat, smooth
bottom. Here we do not vary the box width, we just increase
(or decrease) the diameters 
of the
left- and rightmost particles in row M=0. All other particles
have the fixed diameter 
so that we change the effective width
of the system. In Fig. 7 we plot the vertical
stresses V(0), V(2), V(4)
for 
piles with 
.
We find that the dip vanishes already for slightly increasing c
and relate the existence of the dip to the presence of open horizontal
bonds in the center of the pile at X = 0.5.
For decreasing c we still observe a dip structure in the pile
but when c gets too small, the stress in the array of particles
may get asymmetric, since the pefect triangular arangement
is disturbed (see c=-2/15).
Figure 7:
Vertical stresses V(0), V(2), V(4),
in rows M=0, 2, and 4 respectively (from top to bottom)
vs. X for a
pile with smooth, flat bottom and L=22. The two outermost particles
are fixed by vertical walls and have diameter 
.
The inserts gives the relative change c. (a) Large boundary particles
c > 1, and (b) small boundary particles c < 1.