Interaction among inductors:

Practically in electrical circuits, there is almost some mutual inductance between or among coils when they are wound in the cylindrical shape. The magnetic fields extend significantly outside solenoidal coils, and the mutual effects are inevitable. The same is true between and among the lengths of wire, at very-high, ultra-high, and microwave radio frequencies. At times, mutual inductance is all right, and does not have a detrimental effect on behavior of a circuit.

Mutual inductance is minimized by using shielded wires and toroidal inductors. The common shielded wire is coaxial cable.

Coefficient of coupling

The coefficient of coupling, represented by letter k, is a number ranging from 0 (no coupling) to 1. Two coils which are separated by a sheet of solid iron would have essentially k = 0; 2 coils wound on the same form, one right over the other, would have k = 1 practically.

Mutual  inductance

The mutual inductance is specified by the letter M and is expressed in the same units as inductance: henrys, microhenrys, millihenrys, or nanohenrys. The value of M is the function of values of the inductors, and of the coefficient of coupling also.

For the 2 inductors,  having values of L1 and L2 (both are expressed in same size units), and having coefficient of coupling k, the mutual inductance M can be found by multiplying the inductance values, taking square root of result, and then multiplying it by k. Mathematically,

M = k (L1L2)^1/2