The displacement amplitude coefficients, An are also formally specified/determined by the
initial conditions (i.e. harmonic content) of the vibrating string at time t = 0. Physically, this
means that the detailed shape/configuration of the vibrating string at time t = 0, e.g. a triangle,
sawtooth or square wave-shape, etc. completely specifies – by the method of Fourier analysis
(i.e. harmonic analysis) – the exact harmonic content (i.e. allowed fn values), the harmonic
amplitudes (values of An) and the phases, ϕn (or δn). For example, for a symmetrical triangletype
standing wave (which has reflection symmetry about its mid-point), only odd-n
coefficients An are non-zero. For an asymmetrical triangle-type standing wave, which does not
have reflection symmetry about its mid-point) both even and odd-n coefficients An are nonzero.
For a 50% duty-cycle type square wave (which also has reflection symmetry about its
mid-point), again only odd-n coefficients An are non-zero. For further details of how this is
accomplished, see e.g. the UIUC P498POM lecture notes on Fourier Analysis, I-IV.