Normalized Jacobi polynomials
Fix docstring of normalized Jacobi polynomial
Issue #3773: Error in acos(x).rewrite(atan)

Fixed acos(x).rewrite(atan) so that it accepts symbolic inputs.
Issue #3773: Added test case to check acos(x).rewrite(atan)

Added a test case to test_trigonometric to make sure
acos(x).rewrite(atan) produces the correct formula 2*atan(sqrt(1 -
x**2)/(1 + x))
Changed acos.rewrite(atan) to be more robust

The previous implementation of acos(x).rewrite(atan) produced outputs
which did not match with corresponding values from acos(x). A more
robust formula is used here to accommodate these errors.
Changed the test for acos(x).rewrite(atan)

Changed the test for acos(x).rewrite(atan) to accommodate the new
rewrite function
Euler totient function as symbolic SymPy class
Merge pull request #2033 from v9joshi/3773_acos

Issue #3773: Error in acos(x).rewrite(atan)
Add Varun Joshi to the AUTHORS/aboutus. Welcome to SymPy!
apart and cancel are nc-aware
2699: use xthreaded, not threaded in apart
modify xthreaded and xreplace to fail gracefully

- xreplace leave args that don't implement xreplace alone
- xtrheaded allows objects that don't rebuild to return unchanged
Hints now return eq also
Merge pull request #2009 from raoulb/jacobi_poly_norm

Normalized Jacobi polynomials
Merge pull request #2043 from Manoj-Kumar-S/bug

Fixes Issue 3780
Merge pull request #2036 from hacman/totient

Symbolic Totient
modifications to simplify

== add quick exit for simplify if expr is Atom

== 3712: try return factored expression more often

== 3730: use as_content_primitive and cancel
Merge pull request #1925 from smichr/3712

3712 - simplify changes
upgrade solving for abs, powers, and real/imag variables

Also:

    use powdenest before factoring in _tsolve

    fix detection of difference of functions
add bivariate solving to solvers