Usually real-world practices are real functionIf x is real → a−k=ak∗a_{-k} = a_k^\asta−k=ak∗ → ∣a−k∣=∣ak∣|a_{-k}| = |a_k|∣a−k∣=∣ak∣ (absolute value is a2+b2\sqrt{a^2 + b^2}a2+b2 when a+bja +bja+bj)If x is real and even → ak=a−k=ak∗a_k = a_{-k} = a_k^\astak=a−k=ak∗ → Fourier coefficient are real and evenIf x is real and odd → ak=−a−k=−ak∗a_k = -a_{-k} = -a_k^\astak=−a−k=−ak∗ → Fourier coefficient are purely imaginary and odd