Formation of $\pi \mu$ atoms in $K_{\mu 4} decay

We derive the decay rate of $\pi \mu$ atom formation in $K_{\mu 4}$ decay. Using the obtained expressions we calculate the decay rate of atom formation and point out that considered decay can give a noticeable contribution as a background to the fundamental decay $K^{+}\to {\pi }^{+}\nu \bar{\nu }$.