[doc] add instructions for sign_check_error

doc/source/generated_cpp.txt

@@ -169,7 +169,8 @@ A program demonstrating the use of the C++ library is generated for each integra
     of integration variables upon which they depend.
     The real and complex parameters are bound to the values passed in `real_parameters` and `complex_parameters`.
     If enabled, contour deformation is controlled by the parameters `number_of_presamples`, `deformation_parameters_maximum`, `deformation_parameters_minimum`, `deformation_parameters_decrease_factor`
-    which are documented in :class:`pySecDec.integral_interface.IntegralLibrary`.
+    which are documented in :class:`pySecDec.integral_interface.IntegralLibrary`. In case of a sign check error (sign_check_error), manually set `number_of_presamples`, `deformation_parameters_maximum`,
+    and `deformation_parameters_minimum`.
 
     Passing the `integrand_t` to the :cpp:func:`secdecutil::Integrator::integrate` function of an instance of a particular :cpp:class:`secdecutil::Integrator` will return the numerically evaluated integral.
     To integrate all orders of all sectors :cpp:func:`secdecutil::deep_apply` can be used.

doc/source/gettingstarted.rst

@@ -266,6 +266,8 @@ can be added to the integrator argument list:
     # integrate
     str_integral_without_prefactor, str_prefactor, str_integral_with_prefactor = box1L(real_parameters=[4.0, -0.75, 1.25, 1.0])
 
+In case of a sign check error (sign_check_error), the arguments ``number_of_presamples``, ``deformation_parameters_maximum``, and ``deformation_parameters_minimum`` as described in
+:class:`IntegralLibrary <pySecDec.integral_interface.IntegralLibrary>` can be used to modify the contour.
 At this point the string ``str_integral_with_prefactor`` contains the full result of the integral and can be manipulated as required.
 In the ``integrate_box1L.py`` an example is shown how to parse the expression with `sympy` and access individual orders of the regulator.
 
@@ -319,7 +321,8 @@ The :cpp:func:`name::make_integrands` function returns an :cpp:class:`secdecutil
     //  Generate the integrands (optimization of the contour if applicable)
         const std::vector<box1L::nested_series_t<box1L::integrand_t>> sector_integrands = box1L::make_integrands(real_parameters, complex_parameters);
 
-The contour deformation can be adjusted via additional arguments to :cpp:func:`name::make_integrands`. The sectors can be added before integration::
+The contour deformation has to be adjusted in case of a sign check error (sign_check_error). This can be done via additional arguments to :cpp:func:`name::make_integrands`.
+The sectors can be added before integration::
 
     //  Add integrands of sectors (together flag)
         const box1L::nested_series_t<box1L::integrand_t> all_sectors = std::accumulate(++sector_integrands.begin(), sector_integrands.end(), *sector_integrands.begin() );

pySecDec/integral_interface.py

@@ -521,6 +521,8 @@ class IntegralLibrary(object):
         unsigned int, optional;
         The number of samples used for the
         contour optimization.
+        A larger value here may resolve a sign
+        check error (sign_check_error).
         This option is ignored if the integral
         library was created without deformation.
         Default: ``100000``.
@@ -529,6 +531,8 @@ class IntegralLibrary(object):
         float, optional;
         The maximal value the deformation parameters
         :math:`\lambda_i` can obtain.
+        Lower this value if you get a sign check
+        error (sign_check_error).
         If ``number_of_presamples=0``, all
         :math:`\lambda_i` are set to this value.
         This option is ignored if the integral
@@ -539,6 +543,8 @@ class IntegralLibrary(object):
         float, optional;
         The minimal value the deformation parameters
         :math:`\lambda_i` can obtain.
+        Lower this value if you get a sign check
+        error (sign_check_error).
         If ``number_of_presamples=0``, all
         :math:`\lambda_i` are set to this value.
         This option is ignored if the integral
@@ -548,9 +554,14 @@ class IntegralLibrary(object):
     :param deformation_parameters_decrease_factor:
         float, optional;
         If the sign check with the optimized
-        :math:`\lambda_i` fails, all :math:`\lambda_i`
-        are multiplied by this value until the sign
-        check passes.
+        :math:`\lambda_i` fails during the presampling
+        stage, all :math:`\lambda_i` are multiplied
+        by this value until the sign check passes.
+        We recommend to rather change
+        ``number_of_presamples``,
+        ``deformation_parameters_maximum``,
+        and ``deformation_parameters_minimum``
+        in case of a sign check error.
         This option is ignored if the integral
         library was created without deformation.
         Default: ``0.9``.