= Karatsuba digit-operation cost
{c}
{title2=$T(n)=O(n^{\log_2 3})$}
Count a fixed-base digit <addition>, <subtraction>, digit copy, or single-digit <multiplication> as constant cost. Base $B$ stays fixed as $n$ grows; an entire arbitrary-length <integer> <multiplication> is not one constant-cost step. With this model, splitting, sum formation, shifts and recombination require $O(n)$ digit operations. Ordinary digit-by-digit <multiplication> forms $n^2$ digit products for two $n$-position operands. The GMP base-case description uses this rectangular product arrangement.<GNU MP 6.3.0 manual/Basecase Multiplication>[GNU MP 6.3.0 manual, §15.1.1 Basecase Multiplication: rectangular cross-products and O(NM) cost]{ref}
The ideal equal-half <recurrence relation> is
$$
T(n)=3T(n/2)+O(n).
$$
For $n=2^k$, level $j$ has $3^j$ calls of size $n/2^j$. If each internal call incurs exactly $\alpha$ times its size in additional digit work, that level contributes $\alpha n(3/2)^j$. There are $3^k=n^{\log_2 3}$ leaves. The <geometric series> of internal work is
$$
\alpha n\sum_{j=0}^{k-1}(3/2)^j=2\alpha(3^k-2^k).
$$
Thus fixed positive leaf cost plus this internal work grows on the order of $n^{\log_2 3}$. In <Big O notation>, $T(n)=O(n^{\log_2 3})$. Removing one of four products at every level matters much more than saving just one at the root.
\Image[/karatsuba-recursion-digit-cost.png]
{title=Karatsuba recursion levels and an explicitly stipulated cost comparison}
{description=Left: the ideal equal-half <recursion> for $n=8$, with three children per internal call. Right: model counts for $n=2^k$, one unit per leaf and internal overhead $\alpha s$ for size $s$ with $\alpha=1$. The resulting count is $3\cdot3^k-2\cdot2^k$, compared with $n^2$ ordinary digit products. This illustrates the <recurrence relation>; its constants and crossover are stipulated, rather than measured execution times.}
The literal sum-based implementation instead admits the upper bound
$$
T(n)\leq3T(\lceil n/2\rceil+1)+Cn
$$
for a nondecreasing worst-case cost envelope above a fixed base-case threshold. The extra digit does not change the exponent: since $\lceil n/2\rceil+1\leq n/2+3/2$, subtracting three from the size makes each child's adjusted size at most half its parent's. Stop this comparison at any fixed threshold greater than three; its <recursion> depth is $\log_2 n+O(1)$ and the level-work sum is bounded by the same <geometric series> up to constant factors. Exact call counts can depend on carries, zero shortcuts and the threshold, so they need not equal the ideal $3^j$ at each depth.
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