built-in, whole, entire, inbuilt, intrinsic, inherent, intrinsical, constitutional, intact, essential, more...

inessential, extraneous, nonessential, irrelevant, dispensable

Riemann, Lebesgue, Euler, Nilakantha, Gauss, Green, Stokes, Cauchy, more...

first, definite, second, fourier, particular, double, overlap, indefinite, complete, last, functional

essential part,   important part of,   part and parcel of,   essential part of,   necessary member,   forming a whole,   fundamental part of