function besy0 (x)
c august 1980 version.  w. fullerton, c3, los alamos scientific lab.
      dimension by0cs(13), bm0cs(21), bth0cs(24)
      external alog, besj0, csevl, inits, r1mach, sin, sqrt
c
c series for by0        on the interval  0.          to  1.60000d+01
c                                        with weighted error   1.20e-17
c                                         log weighted error  16.92
c                               significant figures required  16.15
c                                    decimal places required  17.48
c
      data by0 cs( 1) /   -.0112778393 92865573e0 /
      data by0 cs( 2) /   -.1283452375 6042035e0 /
      data by0 cs( 3) /   -.1043788479 9794249e0 /
      data by0 cs( 4) /    .0236627491 83969695e0 /
      data by0 cs( 5) /   -.0020903916 47700486e0 /
      data by0 cs( 6) /    .0001039754 53939057e0 /
      data by0 cs( 7) /   -.0000033697 47162423e0 /
      data by0 cs( 8) /    .0000000772 93842676e0 /
      data by0 cs( 9) /   -.0000000013 24976772e0 /
      data by0 cs(10) /    .0000000000 17648232e0 /
      data by0 cs(11) /   -.0000000000 00188105e0 /
      data by0 cs(12) /    .0000000000 00001641e0 /
      data by0 cs(13) /   -.0000000000 00000011e0 /
c
c series for bm0        on the interval  0.          to  6.25000d-02
c                                        with weighted error   4.98e-17
c                                         log weighted error  16.30
c                               significant figures required  14.97
c                                    decimal places required  16.96
c
      data bm0 cs( 1) /    .0928496163 7381644e0 /
      data bm0 cs( 2) /   -.0014298770 7403484e0 /
      data bm0 cs( 3) /    .0000283057 9271257e0 /
      data bm0 cs( 4) /   -.0000014330 0611424e0 /
      data bm0 cs( 5) /    .0000001202 8628046e0 /
      data bm0 cs( 6) /   -.0000000139 7113013e0 /
      data bm0 cs( 7) /    .0000000020 4076188e0 /
      data bm0 cs( 8) /   -.0000000003 5399669e0 /
      data bm0 cs( 9) /    .0000000000 7024759e0 /
      data bm0 cs(10) /   -.0000000000 1554107e0 /
      data bm0 cs(11) /    .0000000000 0376226e0 /
      data bm0 cs(12) /   -.0000000000 0098282e0 /
      data bm0 cs(13) /    .0000000000 0027408e0 /
      data bm0 cs(14) /   -.0000000000 0008091e0 /
      data bm0 cs(15) /    .0000000000 0002511e0 /
      data bm0 cs(16) /   -.0000000000 0000814e0 /
      data bm0 cs(17) /    .0000000000 0000275e0 /
      data bm0 cs(18) /   -.0000000000 0000096e0 /
      data bm0 cs(19) /    .0000000000 0000034e0 /
      data bm0 cs(20) /   -.0000000000 0000012e0 /
      data bm0 cs(21) /    .0000000000 0000004e0 /
c
c series for bth0       on the interval  0.          to  6.25000d-02
c                                        with weighted error   3.67e-17
c                                         log weighted error  16.44
c                               significant figures required  15.53
c                                    decimal places required  17.13
c
      data bth0cs( 1) /   -.2463916377 4300119e0 /
      data bth0cs( 2) /    .0017370983 07508963e0 /
      data bth0cs( 3) /   -.0000621836 33402968e0 /
      data bth0cs( 4) /    .0000043680 50165742e0 /
      data bth0cs( 5) /   -.0000004560 93019869e0 /
      data bth0cs( 6) /    .0000000621 97400101e0 /
      data bth0cs( 7) /   -.0000000103 00442889e0 /
      data bth0cs( 8) /    .0000000019 79526776e0 /
      data bth0cs( 9) /   -.0000000004 28198396e0 /
      data bth0cs(10) /    .0000000001 02035840e0 /
      data bth0cs(11) /   -.0000000000 26363898e0 /
      data bth0cs(12) /    .0000000000 07297935e0 /
      data bth0cs(13) /   -.0000000000 02144188e0 /
      data bth0cs(14) /    .0000000000 00663693e0 /
      data bth0cs(15) /   -.0000000000 00215126e0 /
      data bth0cs(16) /    .0000000000 00072659e0 /
      data bth0cs(17) /   -.0000000000 00025465e0 /
      data bth0cs(18) /    .0000000000 00009229e0 /
      data bth0cs(19) /   -.0000000000 00003448e0 /
      data bth0cs(20) /    .0000000000 00001325e0 /
      data bth0cs(21) /   -.0000000000 00000522e0 /
      data bth0cs(22) /    .0000000000 00000210e0 /
      data bth0cs(23) /   -.0000000000 00000087e0 /
      data bth0cs(24) /    .0000000000 00000036e0 /
c
c twodpi = 2.0/pi
      data twodpi / 0.6366197723 6758134e0 /
      data pi4 / 0.7853981633 9744831e0 /
      data alnhaf / -0.6931471805 59945309e0 /
      data nty0, ntm0, ntth0, xsml, xmax / 3*0, 2*0./
c
      if (nty0.ne.0) go to 10
      nty0 = inits (by0cs, 13, 0.1*r1mach(3))
      ntm0 = inits (bm0cs, 21, 0.1*r1mach(3))
      ntth0 = inits (bth0cs, 24, 0.1*r1mach(3))
c
      xsml = sqrt (4.0*r1mach(3))
      xmax = 1.0/r1mach(4)
c
 10   if (x.le.0.) call seteru (29hbesy0   x is zero or negative, 29,
     1  1, 2)
      if (x.gt.4.0) go to 20
c
      y = 0.
      if (x.gt.xsml) y = x*x
      besy0 = twodpi*(alnhaf+alog(x))*besj0(x) + .375 +
     1  csevl (.125*y-1., by0cs, nty0)
      return
c
 20   if (x.gt.xmax) call seteru (
     1  37hbesy0   no precision because x is big, 37, 2, 2)
c
      z = 32.0/x**2 - 1.0
      ampl = (0.75 + csevl (z, bm0cs, ntm0)) / sqrt(x)
      theta = x - pi4 + csevl (z, bth0cs, ntth0) / x
      besy0 = ampl * sin (theta)
c
      return
      end