# Where is the mistake in this apparently-O(n lg n) multiplication algorithm?

A recent puzzle blog post about finding three evenly spaced ones lead me to a stackoverflow question with a top answer that claims to do it in O(n lg n) time. The interesting part is that the solution involves squaring a polynomial, referencing a paper that describes how to do it in O(n lg n) time.

Now, multiplying polynomials is practically the same as multiplying numbers. The only real difference is the lack of carries. But... the carries can also be done in O(n lg n) time. For example:

    var value = 100; // = 0b1100100

var inputBitCount = value.BitCount(); // 7 (because 2^7 > 100 >= 2^6)
var n = inputBitCount * 2; // 14
var lgn = n.BitCount(); // 4 (because 2^4 > 14 => 2^3)
var c = lgn + 1; //5; enough space for 2n carries without overflowing

// do apparently O(n log n) polynomial multiplication
var p = ToPolynomialWhereBitsAreCoefficients(value); // x^6 + x^5 + x^2
var p2 = SquarePolynomialInNLogNUsingFFT(p); // x^12 + 2x^11 + 2x^10 + x^8 + 2x^7 + x^4
var s = CoefficientsOfPolynomial(p2); // [0,0,0,0,1,0,0,2,1,0,2,2,1]
// note: s takes O(n lg n) space to store (each value requires at most c-1 bits)

// propagate carries in O(n c) = O(n lg n) time
for (var i = 0; i < n; i++)
for (var j = 1; j < c; j++)
if (s[i].Bit(j))
s[i + j].IncrementInPlace();

// extract bits of result (in little endian order)
var r = new bool[n];
for (var i = 0; i < n; i++)
r[i] = s[i].Bit(0);

// r encodes 0b10011100010000 = 10000


So my question is this: where's the mistake, here? Multiplying numbers in O(n lg n) is a gigantic open problem in computer science, and I really really doubt the answer would be this simple.

• Is the carrying wrong, or not O(n lg n)? I've worked out that lg n + 1 bits per value is enough to track the carries, and the algorithm is so simple I'd be surprised if it was wrong. Note that, although an individual increment can take O(lg n) time, the aggregate cost for x increments is O(x).
• Is the polynomial multiplication algorithm from the paper wrong, or have conditions that I'm violating? The paper uses a fast fourier transform instead of a number theoretic transform, which could be an issue.
• Have a lot of smart people missed an obvious variant of the Schönhage–Strassen algorithm for 40 years? This seems by far the least likely.

I've actually written code to implement this, except for the efficient polynomial multiplication (I don't understand the number theoretic transform well enough yet). Random testing appears to confirm the algorithm being correct, so the issue is likely in the time complexity analysis.

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Shouldn't the square include x^10 + 2x^8 ? x^10 only once (x^5 * x^5), and x^8 twice (x^6 * x^2 + x^2 * x^6) – Sjoerd Jan 19 '13 at 17:00
I did the example by hand. I made an arithmetic mistake. Sorry. I did actually implement the algorithm and test it and get correct results, though. – Craig Gidney Jan 19 '13 at 19:34

Your algorithm is very similar to Schönhage–Strassen. In the FFT step, there are large numbers involved - as you mention, their size can be up to $O(\log n)$. Arithmetic on them doesn't come for free. You need to apply the construction recursively, and you'll lose something.