Any of the seven mathematical expressions that are undefined:

0/0, /∞, ∞·0, 1, 00, ∞0, ∞ - ∞

There are a few trigonometric expressions that are also undefined, but they are mostly 'divide by zero' things. They are:

tan 90°, sec 90°, cosec 0°, cot

Also, in the number set R (all real numbers), the square root of any negative number is undefined; and in any number set (N,J,Q,R,C), you cannot yet divide by zero.

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