Etymology
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subset (n.)
also sub-set, "subordinate set," 1897, originally in mathematics, from sub- + set (n.1).
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rotor (n.)

1873, an irregular shortening of rotator, originally in mathematics. Mechanical sense of "rotating part of a motor" is attested by 1903; specifically of helicopters from 1930.

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factor (v.)
1610s, "act as an agent, manage," from factor (n.). The use in mathematics is attested from 1837. Related: Factored; factoring.
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radicand (n.)

in mathematics, the number under a radical sign, by 1843, from German, from Modern Latin radicandus, gerundive of radicare "to take root," from radix "root" (from PIE root *wrād- "branch, root").

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residual (n.)

1550s, in mathematics, "a residual quantity," from residual (adj.) or from residue + -al (2). Residuals "royalties for repeated performance or broadcast" is attested by 1960.

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-ics 
in the names of sciences or disciplines (acoustics, aerobics, economics, etc.), a 16c. revival of the classical custom of using the neuter plural of adjectives with Greek -ikos "pertaining to" (see -ic) to mean "matters relevant to" and also as the titles of treatises about them. Subject matters that acquired their English names before c. 1500, however, tend to be singular in form (arithmetic, logic, magic, music, rhetoric). The grammatical number of words in -ics (mathematics is/mathematics are) is a confused question.
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analogical (adj.)
"done by or of the nature of an analogy," 1580s in mathematics; c. 1600 in general use; see analogy + -ical.
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frustum (n.)
"remaining piece after a part has been cut off," 1650s, in mathematics, from Latin frustum "piece broken off," from PIE *bhrus-to-, from root *bhreu- "to cut, break up" (see bruise (v.)).
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invariant (adj.)

"remaining always the same, not varying or changing," 1795, from in- (1) "not" + variant (adj.). As a noun, in mathematics, from 1851. Related: Invariance.

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numerate (adj.)

early 15c., "numbered, counted," from Latin numeratus "counted out," past participle of numerare "to count, to number," from numerus "a number" (see number (n.)). By 1959 as "acquainted with the basic principles of mathematics" (see numeracy).

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