subroutine poismu(ap,rho,ibc,rmu,dtt)
c
c Solves a-dtt*rmu*delsq(a)=rho for a
c assuming a(ld,m,n)=0.0
c
c     if ibc=1 then (d/dx)ap = 0.0 at r=0.0
c     if ibc=-1 then  ap = 0.0 at r=0.0
c
      implicit none
      include 'itg.par'
      real ap(lz,mz,nz,nspecz),rho(lz,mz,nz,nspecz),
     * e(lz),f(lz),dtr,rmu,dtt,d,den
      integer ibc,mr2,lm,lp,l,m,n,mr,ll,i
      include 'itg.cmn'
c
      dtr=dtt*rmu
c
      do 10 i=1,nspecies
      do 10 n=1,nd
      do 3 m=1,md
      mr=mrr(m,n)
      mr2=mr**2
      if(ibc.eq.1) then
      den=1.0 + 0.5*r(2)*r(2)*(mr2/(y0*y0)+1.0/dtr)
      e(1)=1.0/den
      f(1)=0.5*r(2)*r(2)*rho(1,m,n,i)/(dtr*den)
      else
      e(1)=0.
      f(1)=0.
      endif
    6 continue
      do 1 l=2,ldb
      lm=l-1
      d=eb(l)+fb(l)+mr2/(y0*y0)+1.0/dtr-fb(l)*e(lm)
      e(l)=eb(l)/d
      f(l)=(rho(l,m,n,i)/dtr+fb(l)*f(lm))/d
1     continue
      ap(ld,m,n,i)=0.0
      do 2 ll=1,ldb
      l=ld-ll
      lp=l+1
      ap(l,m,n,i)=e(l)*ap(lp,m,n,i)+f(l)
2     continue
3     continue
10    continue
      return
      end