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Grothendieck’s Constant - mklabgr - 08-15-2026
Grothendieck’s Constant
Summary
The MathWorld article introduces Grothendieck’s constant, a remarkable universal constant arising from Alexander Grothendieck’s 1953 work in functional analysis. Roughly speaking, Grothendieck discovered that when a certain bilinear expression involving ordinary bounded real numbers is replaced by one involving inner products of vectors in a Hilbert space, the resulting expression can grow—but only by a fixed multiplicative factor, independent of the dimension. For an $n\times n$ real matrix this factor is denoted $k_R(n)$, and the limiting quantity $k_R=\lim_{n\to\infty}k_R(n)$ is usually called the real Grothendieck constant, also written $K_G$. The remarkable point is that such a dimension-independent bound exists at all.
Despite decades of research, the exact value of $K_G$ remains unknown. MathWorld gives the classical bounds $1.67696\ldots\leq K_G\leq1.7822139781\ldots$. Jean-Louis Krivine conjectured that the upper bound was exact, proposing $K_G=\frac{\pi}{2\ln(1+\sqrt2)}\approx1.7822139$, but this long-standing conjecture was disproved in 2011 by Mark Braverman, Konstantin Makarychev, Yury Makarychev and Assaf Naor, who showed that $K_G$ is strictly smaller than Krivine’s value. For finite dimensions more can be said—for example, $k_R(2)=\sqrt2$, while only ranges are known for several higher dimensions.
The article also describes the complex Grothendieck constant, obtained when the matrix entries and scalar variables are complex. Its limiting value $k_C$ is likewise not known exactly; MathWorld reports $1.33807\leq k_C\leq1.40491$ and discusses Haagerup’s proposed value of approximately $1.4045759$. Thus Grothendieck’s constant is an excellent example of a mathematical object whose existence and importance are well established while its precise numerical value remains elusive. What began in abstract Banach-space theory has also become important in areas such as optimization, approximation algorithms, graph problems, communication complexity and quantum-information theory.
Key Takeaways
- Universal bound: Grothendieck’s inequality guarantees a constant controlling the passage from scalar bilinear expressions to vector inner products, regardless of dimension.
- Exact value unknown: The real constant satisfies approximately $1.67696\leq K_G<1.78221$.
- Famous conjecture overturned: Krivine’s proposed exact value was disproved in 2011.
- Broad significance: A constant originating in functional analysis now connects pure mathematics with optimization, theoretical computer science and quantum-information problems.
ARTICLE