I will be assuming they are using the relation operator for assignment rather than equality, as the former has more interesting consequences.

With a little effort we can come up with say,

(-(1/30))n^5+(5/6)n^4-(49/6)n^3+(247/6)n^2-(459/5)n+84 which results in,

18, 32, 50, 72, 98, 124, 138, 116, . . . .

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But,

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2n^2 gives

18, 32, 50, 72, 98, 128, 162, 200, . . . .

(The popular choice)

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However,

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n^5-25n^4+245n^3-1173n^2+2754n-2520 gives

18, 32, 50, 72, 98, 248, 882, 2720, . . . .

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But then there is,

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(8/5)n^5-40n^4+392n^3-1878n^2+(22032/5)n-4032 that gives

18, 32, 50, 72, 98, 320, 1314, 4232, . . . .

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And then there is ....

... I can keep going all day, but I'll just stop there.

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All these work, take your pick :)

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Finally, the point also is that given any finite set of numbers, the set does not necessarily define any single sequence.