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Resolve[ForAll[n,Element[n,Integers],!IntegerQ[Sqrt[n^2-7 n-1]]]]

True

But as I know,when the n is $17$,Sqrt[n^2 - 7 n - 1] is $13$(integer).What's happen of this function?

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  • $\begingroup$ ForAll[] doesn't work that way. Resolve[ForAll[n, Element[n, Integers], ! IntegerQ[n]]] also evaluates to True. $\endgroup$
    – Feyre
    Feb 13, 2017 at 13:17
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    $\begingroup$ @Feyre Funny,ForAll[n, ! IntegerQ[n]] will give True,too.Why? $\endgroup$
    – yode
    Feb 13, 2017 at 13:21

1 Answer 1

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All functions ending in Q evaluate to True or False immediately. Therefore IntegerQ[x] is False if x has no value.

You are essentially writing

Resolve[ForAll[n, Element[n, Integers], True]]

so you get True.

IntegerQ is a "programming function" that tests the data type of the argument. It is not a "mathematical function" that will stay unevaluated with symbolic arguments and can represent a mathematical statement.

Use something \[Element] Integers instead.

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  • $\begingroup$ So how to write the code will be my original intention? $\endgroup$
    – yode
    Feb 13, 2017 at 14:12
  • $\begingroup$ @yode Use Element or NotElement, as I said in the last line of the answer. Mathematica will not be able to solve this problem though. $\endgroup$
    – Szabolcs
    Feb 13, 2017 at 14:28
  • $\begingroup$ Good lesson to me.Thanks. $\endgroup$
    – yode
    Feb 13, 2017 at 14:36

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