What does strength refer to in mathematics?
Suppose you have a theorem that says "If $X$, then $Y$." There are two ways to strengthen such a theorem:
- Assume less. If you can reduce the number of hypotheses, but still prove the same conclusion, then you have proved a more "powerful" result (in the sense that it applies in more situations).
- Prove more. If you can keep the same hypotheses, but add more information to the conclusion, then you have also produced a more "powerful" result.
Here is an easy example from Geometry.
Let $ABCD$ be a (non-square) rectangle. Then the internal angle bisectors of the vertices intersect at four points $WXYZ$, which are the vertices of a rectangle.
(You need the condition that $ABCD$ is not a square because if it is a square then all four angle bisectors coincide at a single point.)
Here are a few ways to strengthen the theorem:
- The hypothesis "$ABCD$ is a (non-square) rectangle" can be relaxed to the more general "$ABCD$ is a (non-rhombic) parallelogram". The conclusion that $WXYZ$ is a rectangle still holds.
- Alternatively, you can keep the original hypothesis that $ABCD$ is a (non-square) rectangle, and strengthen to the conclusion to say that $WXYZ$ is not just a rectangle, but a square.
- Having done that, you can then strengthen the conclusion of the theorem even more, by noting that the diagonal of square $WXYZ$ is equal in length to the difference of the lengths of the sides of $ABCD$.
- Once you know that, you can now strengthen the theorem even more by (finally) removing the hypothesis that $ABCD$ is non-square, and including the case in which the four angle bisectors coincide at a single point as forming a "degenerate" square with a diagonal of length zero.
Claims are said to be stronger or weaker, depending on the amount of information you can imply from the claim. For example, if $x$ is a positive solution of the equation $x^2 = 2$, then the claim $x > 0$ is weaker than $x > 1$ (the second is more precise).
Sometimes we try to make these implications strongest possible. For example, $1$ is the strongest integer lower bound on such an $x$.
In logic, saying that $\text{statement }x\text{ is stronger than statement }y$, is equivalent to saying that:
$$[\text{statement }x\text{ implies statement }y]\text{ but [statement }y\text{ does not imply statement }x]$$
For example, suppose we conjecture the following statements on a set $S$:
- Conjecture $x$: all the elements in $S$ are divisible by $4$
- Conjecture $y$: all the elements in $S$ are divisible by $2$
It is obvious that conjecture $x$ is stronger than conjecture $y$.