Spin resonance functions a^ν_U/ν_L(x,p) for the direct resonances D1 – D3 and the frequency ratios n:m = 5:4 (black solid line), 4:3 (black dashed line), 3:2 (black dotted line), 5:3 (red solid line), 2:1 (red dashed line), 5:2 (red dotted line), 3:1 (blue solid line), 4:1 (blue dashed line), 5:1 (blue dotted line). Black thick line represents a_ms(x), which implicitly determines the radius of the marginally stable orbit x_ms giving a natural restriction on the validity of the resonance model versions involving radial epicyclic oscillations. In the case D3, the black points determine turning points of the functions a^ν_U/ν_L(x,p) joining two separate branches of the solution of the resonance condition equation. Of course, only those parts of the resonance functions with x > x_ms(a) are acceptable.