real variable function

understanding, comprehension / appreciation

truth; reality; sincerity / fidelity; faith / kindness / substance; essence / (mathematics) dividend

base, basis, foundation, ground, root / (construction, architecture) foundation

to study / to learn

the above / the top (position or direction) / the surface of / superior in skill, age, level, etc.

in addition; furthermore / on top of (which)

understand, comprehend / appreciate

important, significant

importance, consequence, significance