Strange result - Bug in diff(F_tenson, b) ? (least squares - motive)

[Станислав Иванов]

>>> from __future__ import division ... ... from sympy import * ... from sympy.tensor import IndexedBase, Idx ... ... a, b = symbols('a b') ... F_tensor = symbols('F_tensor', cls=Function)

... ... i = map(Idx, ['i']) ... ... x = IndexedBase('x') ... y = IndexedBase('y') ...

>>> F_tensor = (y[i] - (b*x[i]+a))**2

>>> F_tensor

(−a−bx[i]+y[i])2

>>> diff(F_tensor, b)

−2(−a−bx[i]+y[i])2 x[i](−a−bx[i]+y[i])−1

[Aaron Meurer]

What's the result that you would expect? I get

In [31]: diff(F_tensor, b)

Out[31]: -2⋅(-a - b⋅x[i] + y[i])⋅x[i]

By the way, your code should just be i = Idx('i'). map returns a list in Python 2 and an iterator in Python 3. 

Aaron Meurer

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