The real Grothendieck constant

Grothendieck's inequality says that replacing signs by unit vectors can increase a bilinear form by at most a universal factor $K_G$. Its exact value has been open since 1953; in 2026 a run of new bounds fixed its first decimal digit, $1.71360 \le K_G \le 1.78212$.

Problem

The real Grothendieck constant $K_G$ is the smallest constant $C$ such that, for all $m, n \ge 1$ and every real matrix $A = (a_{ij}) \in \mathbb{R}^{m \times n}$,

$$\max_{u_i, v_j \in S^{\infty}} \sum_{i=1}^{m} \sum_{j=1}^{n} a_{ij} \langle u_i, v_j \rangle \;\le\; C \max_{\varepsilon_i, \delta_j \in \{-1, 1\}} \sum_{i=1}^{m} \sum_{j=1}^{n} a_{ij}\, \varepsilon_i \delta_j,$$

where $S^{\infty}$ is the unit sphere of a real Hilbert space (equivalently, of $\mathbb{R}^d$ for $d$ large enough). Determine $K_G$, or narrow the known range.

Background

Grothendieck proved the inequality in 1953 in his work on tensor products of Banach spaces, and it has since become a basic tool in functional analysis, operator theory and theoretical computer science. In optimization terms, the left side is a semidefinite relaxation of the $\pm 1$ problem on the right, and $K_G$ is the worst-case ratio between the two. Krivine (1979) proved $K_G \le \pi / (2 \ln(1+\sqrt{2})) \approx 1.782214$ and conjectured that this was the exact value; Braverman, Makarychev, Makarychev and Naor disproved that in 2011. Pisier's survey describes the many forms and uses of the inequality.

What is known

• Lower bounds: $\pi/2$ (Grothendieck, 1953); about $1.67696$ (Davie, 1984, and Reeds, 1991, both unpublished), which stood for four decades; strict improvements by $10^{-26}$ (Heilman, 2026) and $10^{-12}$ (Jones and Malavolta, 2026); then $6\pi/11 \approx 1.71360$ (Saha, Li, Xue, Chaudhuri, Klivans, Kothari and Meka, 2026, preprint).
• Upper bounds: $\sinh(\pi/2) \approx 2.30130$ (Grothendieck); $2.261$ (Rietz, 1974); Krivine's $1.782214$ (1979); strictly below Krivine's value (Braverman et al., 2011); below it by $10^{-5}$ (Heilman, 2026), by $6.039 \times 10^{-5}$ (Li et al., 2026) and by $10^{-4}$, so $K_G < 1.78212$ (Saha et al., 2026). The 2026 bounds are preprints.
• Unverified claim: $K_G \le 1.7813$ (Peng, September 2026), a computer-assisted proof with interval-arithmetic certificates that the source has not yet seen independently replayed.

What would count as progress

• An independent rerun of Peng's certificates for $K_G \le 1.7813$ (degree-11 Hermite threshold, degree-51 correlation polynomial), which would make it an accepted record; or an upper bound below 1.7813 from a new rounding scheme, with a certificate that can be rerun.
• A lower bound above $6\pi/11$. An explicit instance would be a matrix (or kernel) together with a certified value of both sides: a feasible vector solution for the left side and a proof, by exhaustive search for small sizes or by an argument, that no sign choice beats the stated value on the right.
• Any further narrowing of the second decimal digit, which is still open.

References

1. J.-L. Krivine, "Constantes de Grothendieck et fonctions de type positif sur les sphères", Advances in Mathematics 31 (1979), 16–30. doi:10.1016/0001-8708(79)90017-3
2. M. Braverman, K. Makarychev, Y. Makarychev and A. Naor, "The Grothendieck constant is strictly smaller than Krivine's bound", Forum of Mathematics, Pi 1 (2013), e4. doi:10.1017/fmp.2013.4, arXiv:1103.6161
3. G. Pisier, "Grothendieck's theorem, past and present", Bulletin of the AMS 49 (2012), 237–323. doi:10.1090/S0273-0979-2011-01348-9
4. C. Jones and G. Malavolta, "The Grothendieck constant is strictly larger than Davie–Reeds' bound" (2026). arXiv:2603.30039
5. R. Saha, A. Li, A. Xue, S. Chaudhuri, A. Klivans, P. K. Kothari and R. Meka, "New lower and upper bounds for the Grothendieck constant" (2026). arXiv:2608.11158
6. B. Peng, "A computer-assisted upper bound of 1.7813 for the real Grothendieck constant" (2026). arXiv:2609.20074

Source

Imported from Optimization Constants in Mathematics, constant 10a, curated by Terence Tao and contributors. Open according to the source as of 2026-10-20. The statement above is written in our own words; see the source for the original wording, history and discussion.