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torsdag 26 maj 2016

Fatal Attraction of Fundamental Theorem of Calculus?

Calculus books proudly present the Fundamental Theorem of Calculus as the trick of computing an integral
  • I=\int_a^b f(x)dx,
not by tedious summation of little pieces as a Riemann sum
  • \sum_i f(x_i)h_i
on a partition \{x_i\} of the interval (a,b) with step size h_i = x_{i+1} - x_i, but by the formula
  • I = F(b) - F(a)
where F(x) is a primitive function to f(x) satisfying \frac{dF}{dx} = f,

The trick is thus to compute an integral, which by construction is a sum of very many terms, not by doing the summation following the construction, but instead taking just one big leap using a primitive function.

On the other hand, to compute a derivative no trick is needed according to the book; you just compute the derivative using simple rules and a catalog of already computed derivatives.

In a world of analytical mathematics, computing integrals is thus valued higher than computing derivatives, and this is therefore what fills Calculus books.

In a world of computational mathematics, the roles are switched. To compute an integral as a sum can be viewed to be computationally trivial, while computing a derivative \frac{dF}{dx} is a bit more tricky because it involves dividing increments dF by small increments dx.

This connects to Poisson's equation \Delta\phi =\rho of Newton's theory of gravitation discussed in recent posts. What is here to be viewed as given and what is derived? The standard view is that the mass density \rho is given and the gravitational potential \phi is derived from \rho as an integral
  • \phi (x) = \frac{1}{4\pi}\int\frac{\rho (y)}{\vert x-y\vert}dy,
seemingly by instant action at distance. 

In alternative Newtonian gravitation, as discussed in recent posts, we view instead \phi as primordial and \rho =\Delta\phi as being derived by differentiation, with the advantage of requiring only local action.

We thus have two opposing views:
  • putting together = integration requiring (instant) action at distance with dull tool.
  • splitting apart = differentiation involving local action with sharp tool. 
It is not clear what to prefer?

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