In the mathematical study of the differential geometry of surfaces, the Bertrand–Diguet–Puiseux theorem expresses the Gaussian curvature of a surface in terms of the circumference of a geodesic circle, or the area of a geodesic disc. The theorem is named for Joseph Bertrand, Victor Puiseux, and Charles François Diguet.
Let p be a point on a smooth surface M. The geodesic circle of radius r centered at p is the set of all points whose geodesic distance from p is equal to r. Let C(r) denote the circumference of this circle, and A(r) denote the area of the disc contained within the circle. The Bertrand–Diguet–Puiseux theorem asserts that
K
(
p
)
=
lim
r
→
0
+
3
2
π
r
−
C
(
r
)
π
r
3
=
lim
r
→
0
+
12
π
r
2
−
A
(
r
)
π
r
4
.
{\displaystyle K(p)=\lim _{r\to 0^{+}}3{\frac {2\pi r-C(r)}{\pi r^{3}}}=\lim _{r\to 0^{+}}12{\frac {\pi r^{2}-A(r)}{\pi r^{4}}}.}
The theorem is closely related to the Gauss–Bonnet theorem.