Peres metric

${\displaystyle {d\tau }^2=d{t}^2-2f(t+z,x,y)(dt+dz{)}^2-d{x}^2-d{y}^2-d{z}^2}$

for any arbitrary function f. If f is a harmonic function with respect to x and y, then the corresponding Peres metric satisfies the Einstein field equations in vacuum. Such a metric is often studied in the context of gravitational waves. The metric is named for Israeli physicist Asher Peres, who first defined it in 1959.