NMinimize eats all memory b/c of unnecessary symbolic work
- To: mathgroup at smc.vnet.net
- Subject: [mg120720] NMinimize eats all memory b/c of unnecessary symbolic work
- From: Daniel Jensen <jensend at iname.com>
- Date: Sun, 7 Aug 2011 06:15:37 -0400 (EDT)
- Delivered-to: l-mathgroup@mail-archive0.wolfram.com
The following code is a fairly naive way to find the least number whose
square has n divisors (the minimum should be its log and the x_i the
powers in its prime factorization). If I look at the case n00 and use
ten variables instead of twenty, this uses somewhere around 600MB of
memory. With the value of n I'm actually trying to find the answer for,
I need around 20 variables to be sure of not missing the actual
solution, and it quickly uses up all available memory and then thrashes
swap.
n=8*10^6;
a = Table[N[Log[Prime[i]]], {i, 20}];
b = Table[Subscript[x, i], {i, 20}];
cond = Fold[And, Product[2 Subscript[x, i] + 1, {i, 20}] > n,
Table[Subscript[x, i] >= 0, {i, 20}]] && b \[Element] Integers;
NMinimize[{a.b, cond}, b, MaxIterations -> 1000]
It turns out that the problem isn't related to integer programming etc
at all (removing the restriction to the integers doesn't help).
My best guess is that the problem is that Mathematica is wasting all
that memory expanding Product[2 Subscript[x, i] + 1, {i, 20}]. If I
replace the product with just Product[Subscript[x, i],{i,20}] and change
the constraints to be >= 1 rather than 0 I get results without a hassle
and without the kernel using more than 50MB of memory. (Though that
preserves the inequality constraint and doesn't change the task of
minimizing the objective function, it does mess up the integrality
requirement- I get even results, which correspond to half-integers in
the actual problem.)
Searching the Web, I found one person who had a somewhat similar
problem; in their case, they had an objective function which was getting
evaluated symbolically at a huge cost. They were able to remedy it by
making the function only accept numeric input. I don't seem to be able
to do that with the constraint.
Any thoughts on how to fix this?