function n_pairs,area_qso,rmin,rmax,zmin,zmax

;Purpose: to determine the # of pairs per QSO redshift

;Input: redshift 

;Output: number of pairs

;Calling Sequence: n = n_pairs(z)    ;must set z as a single value
;                  print,n

!p.background=!black

area_qso = 4000

zq = findgen(10)/9+.001

zmax = 1.0
zmin = 0.0

rmax = 8
rmin = .001

z = (zmax + zmin)/2.0

d = angdist_de(z)

deltaZ = zmax - zmin

area_circle = !dpi*((rmax^2 - rmin^2)/d^2)*(180/!dpi)^2

n_pairs = area_circle*rho_gal(z)*rho_gal(z)*deltaZ*area_qso

return,n_pairs

end