Sign error in k3 calculation in Bk class

[graemecan]

Hello, I think there is a small error in this line:

Pylians3/library/Pk_library/bispectrum_library.pyx

Line 67 in 8558870

k3 = np.sqrt((k2*np.sin(theta))**2 + (k2*np.cos(theta)+k1)**2)

The sign in front of k1 should be a minus, otherwise this expression doesn't correspond to the cosine rule and the calculated value of k3 is incorrect.

[suicee]

I think the input theta is the angle between k1 and k2, therefore the corresponding angle between k1 and k2 in the triangle is actually (pi-theta). In other words, you can just verify the results using |k3|^2=|k1+k2|^2, this will yield the same formula in this line of code. If your change the sign in front of k1, you are actually calculating |k1-k2|^2. But I guess it's OK if you change the sign here and interpret the input theta as the theta in the triangle. I hope this helps.