Add algorithmic references

Author: rhettinger

Lib/statistics.py

@@ -305,12 +305,14 @@ def _fail_neg(values, errmsg='negative value'):
         yield x
 
 def _isqrt_frac_rto(n: int, m: int) -> float:
-    'Square root of n/m, rounded to the nearest integer using round-to-odd.'
+    """Square root of n/m, rounded to the nearest integer using round-to-odd."""
+    # Refernce: https://www.lri.fr/~melquion/doc/05-imacs17_1-expose.pdf
     a = math.isqrt(n // m)
     return a | (a*a*m != n)
 
 def _sqrt_frac(n: int, m: int) -> float:
-    'Square root of n/m as a float, correctly rounded.'
+    """Square root of n/m as a float, correctly rounded."""
+    # See algorithm sketch at: https://bugs.python.org/msg406911
     # The constant 109 is:  3 + 2 * sys.float_info.mant_dig
     q: int = (n.bit_length() - m.bit_length() - 109) // 2
     if q >= 0: