diff --git a/source/f90/o.f90 b/source/f90/o.f90
new file mode 100644
index 0000000000000000000000000000000000000000..a9f4545dc2766b627f5ff8f208c71440d2a623c5
--- /dev/null
+++ b/source/f90/o.f90
@@ -0,0 +1,23 @@
+double precision function o(x)
+
+! *** rational approximations for ei(x) * exp(-x) + e1(x) * exp(+x)
+! *** in the intervals (0, 1.3) and (1.3, infinity) <accuracy : 4.e-04>
+! *** used for fourier transforming half-lorentzian functions
+
+  implicit none
+
+  double precision, intent(in) :: x
+
+  if ( x < 0 ) then
+    error stop '*** Program abort. Function o is undefined for x < 0 ***' 
+  elseif ( x == 0.0d0 ) then
+    o = 0.0d0
+  elseif ( x <= 1.3d0) then
+    o = -dsinh(x) * dlog(x**2) + x * ((0.03114d0 * x**2 + 0.41666d0) * x**2 + 0.84557d0)
+  else
+    o = (((202.91d0 / x**2 + 932.21d0) / x**2 + 41.740d0) / x**2 + 2.0d0) /  &
+        (((540.88d0 / x**2 + 345.67d0) / x**2 + 18.961d0) / x**2 + 1.0d0) / x
+  endif
+
+  return
+end function o
diff --git a/tests/oxlog/o1.f90 b/tests/oxlog/o1.f90
index 16c9bf04a05f45027c587d7847e544f00e09c88d..a9c9ea77b32378179160b7f175066d4a37e51d13 100644
--- a/tests/oxlog/o1.f90
+++ b/tests/oxlog/o1.f90
@@ -12,5 +12,7 @@ double precision function o1(u)
   u2 = u**2
   o1 = -dsinh(u) * dlog(u2) + u * ((0.03114d0 * u2 + 0.41666d0) * u2 + 0.84557d0)
 
+  write (*,*) u, o1
+
   return
 end function o1
diff --git a/tests/oxlog/o2.f90 b/tests/oxlog/o2.f90
index 76c562b2d807f2e00fe70f38ca4045f44023bd2b..463402e4fe43ded715ea410c47d2313f539c7d97 100644
--- a/tests/oxlog/o2.f90
+++ b/tests/oxlog/o2.f90
@@ -12,6 +12,6 @@ double precision function o2(u)
   u2 = 1 / u**2
   o2 = (((202.91d0 * u2 + 932.21d0) * u2 + 41.740d0) * u2 + 2.0d0) /  &
        (((540.88d0 * u2 + 345.67d0) * u2 + 18.961d0) * u2 + 1.0d0) / u
-
+  write (*,*) u, o2
   return
 end function o2