### Finding extrema and zeros in lists

I create a list Q (below) of pairs of numbers [t1,z1], I want the pairs (maybe just a print of them?) at the local extrema (of z1) and where z1 crosses zero. Is there a simple way to do this?

var('t y z')
P=desolve_system_rk4([z,0.005*exp(-y/8000)*z^2-10],[y,z],ics=[0,32000,0],ivar=t,end_points=400)
Q=[ [t1,z1] for t1,y1,z1 in P]

Thanks, Wayne