The $K \phi$ form factors are investigated at low energies by the method of unitarity bounds adapted so as to include information on the phase and modulus along the elastic region of the unitarity cut. Using as input the values of the form factors at $t = 0$, and at the Callan–Treiman point in the scalar case, stringent constraints are obtained on the slope and curvature parameters of the Taylor expansion at the origin.

Analyticity and unitarity techniques are employed to estimate Taylor coefficients of the pion electromagnetic form factor at $t = 0$ by exploiting the recently evaluated two-pion contribution to the muon $(g − 2)$ and the phase of the pion electromagnetic form factor in the elastic region, known from $\pi \pi$ scattering by Fermi–Watson theorem and the values of the form factor at several points in the space-like region. Regions in the complex 𝑡-plane are isolated where the form factor cannot have zeros.