McCarthy 91 Function - Proof

Proof

Here is a proof that the function behaves as expected.

For 90 ≤ n < 101,

M(n) = M(M(n + 11)) = M(n + 11 - 10), where n + 11 >= 101 since n >= 90 = M(n + 1)

So M(n) = 91 for 90 ≤ n < 101.

We can use this as a base case for induction on blocks of 11 numbers, like so:

Assume that M(n) = 91 for a ≤ n < a + 11.

Then, for any n such that a - 11 ≤ n < a,

M(n) = M(M(n + 11)) = M(91), by hypothesis, since a ≤ n + 11 < a + 11 = 91, by the base case.

Now by induction M(n) = 91 for any n in such a block. There are no holes between the blocks, so M(n) = 91 for n < 101. We can also add n = 101 as a special case.

Read more about this topic:  McCarthy 91 Function

Famous quotes containing the word proof:

    “The thing with Catholicism, the same as all religions, is that it teaches what should be, which seems rather incorrect. This is “what should be.” Now, if you’re taught to live up to a “what should be” that never existed—only an occult superstition, no proof of this “should be”Mthen you can sit on a jury and indict easily, you can cast the first stone, you can burn Adolf Eichmann, like that!”
    —Lenny Bruce (1925–1966)

    “Sculpture and painting are very justly called liberal arts; a lively and strong imagination, together with a just observation, being absolutely necessary to excel in either; which, in my opinion, is by no means the case of music, though called a liberal art, and now in Italy placed even above the other two—a proof of the decline of that country.”
    —Philip Dormer Stanhope, 4th Earl Chesterfield (1694–1773)

    “When children feel good about themselves, it’s like a snowball rolling downhill. They are continually able to recognize and integrate new proof of their value as they grow and mature.”
    —Stephanie Martson (20th century)