double precision function derf (x) c july 1977 edition. w. fullerton, c3, los alamos scientific lab. c last revised june 1983. w. fullerton, imsl, houston. double precision x, erfcs(21), sqeps, sqrtpi, xbig, y, d1mach, 1 dcsevl, derfc, dlog, dsqrt external d1mach, dcsevl, derfc, initds c c series for erf on the interval 0. to 1.00000e+00 c with weighted error 1.28e-32 c log weighted error 31.89 c significant figures required 31.05 c decimal places required 32.55 c data erf cs( 1) / -.4904612123 4691808039 9845440333 76 d-1 / data erf cs( 2) / -.1422612051 0371364237 8247418996 31 d+0 / data erf cs( 3) / +.1003558218 7599795575 7546767129 33 d-1 / data erf cs( 4) / -.5768764699 7674847650 8270255091 67 d-3 / data erf cs( 5) / +.2741993125 2196061034 4221607914 71 d-4 / data erf cs( 6) / -.1104317550 7344507604 1353812959 05 d-5 / data erf cs( 7) / +.3848875542 0345036949 9613114981 74 d-7 / data erf cs( 8) / -.1180858253 3875466969 6317518015 81 d-8 / data erf cs( 9) / +.3233421582 6050909646 4029309533 54 d-10 / data erf cs( 10) / -.7991015947 0045487581 6073747085 95 d-12 / data erf cs( 11) / +.1799072511 3961455611 9672454866 34 d-13 / data erf cs( 12) / -.3718635487 8186926382 3168282094 93 d-15 / data erf cs( 13) / +.7103599003 7142529711 6899083946 66 d-17 / data erf cs( 14) / -.1261245511 9155225832 4954248533 33 d-18 / data erf cs( 15) / +.2091640694 1769294369 1705002666 66 d-20 / data erf cs( 16) / -.3253973102 9314072982 3641600000 00 d-22 / data erf cs( 17) / +.4766867209 7976748332 3733333333 33 d-24 / data erf cs( 18) / -.6598012078 2851343155 1999999999 99 d-26 / data erf cs( 19) / +.8655011469 9637626197 3333333333 33 d-28 / data erf cs( 20) / -.1078892517 7498064213 3333333333 33 d-29 / data erf cs( 21) / +.1281188399 3017002666 6666666666 66 d-31 / c data sqrtpi / 1.772453850 9055160272 9816748334 115d0 / data nterf, xbig, sqeps / 0, 2*0.d0 / c if (nterf.ne.0) go to 10 nterf = initds (erfcs, 21, 0.1*sngl(d1mach(3))) xbig = dsqrt (-dlog(sqrtpi*d1mach(3))) sqeps = dsqrt (2.0d0*d1mach(3)) c 10 y = dabs(x) if (y.gt.1.d0) go to 20 c c erf(x) = 1.0 - erfc(x) for -1.0 .le. x .le. 1.0 c if (y.le.sqeps) derf = 2.0d0*x/sqrtpi if (y.gt.sqeps) derf = x*(1.0d0 + dcsevl (2.d0*x*x-1.d0, 1 erfcs, nterf)) return c c erf(x) = 1.0 - erfc(x) for abs(x) .gt. 1.0 c 20 if (y.le.xbig) derf = dsign (1.0d0-derfc(y), x) if (y.gt.xbig) derf = dsign (1.0d0, x) c return end