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This almost-but-not-quite-circular fashion of defining a sequence of numbers in terms of itself is called a “recursive definition”. This means there is some kind of precise calculational rule for making new elements out of previous ones. The rule might involve adding, multiplying, dividing, whatever — as long as it’s well-defined. The opening gambit of a recursive sequence (in this case, the numbers 1 and 2) can be thought of as a packet of seeds from which a gigantic plant — all of its branches and leaves, infinite in number — grows in a predetermined manner, based on the fixed rule.

The Caspian Gemstones: An Allegory

Leonardo di Pisa’s sequence is brimming with amazing patterns, but unfortunately going into that would throw us far off course. Still, I cannot resist mentioning that 144 jumps out in this list of the first few F numbers because it is a salient perfect square. Aside from 8, which is a cube, and 1, which is a rather degenerate case, no other perfect square, cube, or any other exact power appears in the first few hundred terms of the F sequence.

Several decades ago, people started wondering if the presence of 8 and 144 in the F sequence was due to a reason, or if it was just a “random accident”. Therefore, as computational tools started becoming more and more powerful, they undertook searches. Curiously enough, even with the advent of supercomputers, allowing millions and even billions of F numbers to be churned out, no one ever came across any other perfect powers in Fibonacci’s sequence. The chance of a power turning up very soon in the F sequence was looking slim, but why would a perfect mutual avoidance occur? What do nth powers for arbitrary n have to do with adding up pairs of numbers in Fibonacci’s peculiar recursive fashion? Couldn’t 8 and 144 just be little random glitches? Why couldn’t other little glitches take place?

To cast allegorical light on this, imagine someone chanced one day to fish up a giant diamond, a magnificent ruby, and a tiny pearl at the bottom of the great green Caspian Sea in central Asia, and other seekers of fortune, spurred on by these stunning finds, then started madly dredging the bottom of the world’s largest lake to seek more diamonds, rubies, pearls, emeralds, topazes, etc., but none was found, no matter how much dredging was done. One would naturally wonder if more gems might be hidden down there, but how could one ever know? (Caveat: my allegory is slightly flawed, because we can imagine, at least in principle, a richly financed scientific team someday dredging the lake’s bottom completely, since, though huge, it is finite. For my analogy to be “perfect”, we would have to conceive of the Caspian Sea as infinite. Just stretch your imagination a bit, reader!)

Now the twist. Suppose some mathematically-minded geologist set out to prove that the two exquisite Caspian gems, plus the tiny round pearl, were sui generis — in other words, that there was a precise reason that no other gemstone or pearl of any type or size would ever again, or could ever again, be found in the Caspian Sea. Does seeking such a proof make any sense? How could there be a watertight scientific reason absolutely forbidding any gems — except for one pearl, one ruby, and one diamond — from ever being found on the floor of the Caspian Sea? It sounds absurd.

This is typical of how we think about the physical world — we think of it as being filled with contingent events, facts that could be otherwise, situations that have no fundamental reason for their being as they are. But let me remind you that mathematicians see their pristine, abstract world as the antithesis to the random, accident-filled physical world we all inhabit. Things that happen in the mathematical world strike mathematicians as happening, without any exceptions, for statable, understandable reasons.

This — the Mathematician’s Credo — is the mindset that you have to adopt and embrace if you wish to understand how mathematicians