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  2. Divisibility rule - Wikipedia

    en.wikipedia.org/wiki/Divisibility_rule

    It is divisible by 2 and by 9. [6] 342: it is divisible by 2 and by 9. 19: Add twice the last digit to the rest. (Works because (10a + b) × 2 − 19a = a + 2b; since 19 is a prime and 2 is coprime with 19, a + 2b is divisible by 19 if and only if 10a + b is.) 437: 43 + 7 × 2 = 57. Add 4 times the last two digits to the rest.

  3. Quadratic reciprocity - Wikipedia

    en.wikipedia.org/wiki/Quadratic_reciprocity

    Gauss published the first and second proofs of the law of quadratic reciprocity on arts 125–146 and 262 of Disquisitiones Arithmeticae in 1801.. In number theory, the law of quadratic reciprocity is a theorem about modular arithmetic that gives conditions for the solvability of quadratic equations modulo prime numbers.

  4. List of valid argument forms - Wikipedia

    en.wikipedia.org/wiki/List_of_valid_argument_forms

    It is, however, slightly more complicated than the first two. In short, it states that if one thing happens, another will as well. If that second thing happens, a third will follow it. Therefore, if the first thing happens, it is inevitable that the third will too. [3] It is shown below in logical form. If A, then B If B, then C Therefore if A ...

  5. If and only if - Wikipedia

    en.wikipedia.org/wiki/If_and_only_if

    The corresponding logical symbols are "", "", [6] and , [10] and sometimes "iff".These are usually treated as equivalent. However, some texts of mathematical logic (particularly those on first-order logic, rather than propositional logic) make a distinction between these, in which the first, ↔, is used as a symbol in logic formulas, while ⇔ is used in reasoning about those logic formulas ...

  6. Perfect number - Wikipedia

    en.wikipedia.org/wiki/Perfect_number

    The number of perfect numbers less than n is less than , where c > 0 is a constant. [53] In fact it is (), using little-o notation. [54] Every even perfect number ends in 6 or 28, base ten; and, with the only exception of 6, ends in 1 in base 9.

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  8. Help:Conditional expressions - Wikipedia

    en.wikipedia.org/wiki/Help:Conditional_expressions

    See also: the {{}} template. The #if function selects one of two alternatives based on the truth value of a test string. {{#if: test string | value if true | value if false}} As explained above, a string is considered true if it contains at least one non-whitespace character.

  9. Mathematical induction - Wikipedia

    en.wikipedia.org/wiki/Mathematical_induction

    Then P(0) is true, for if it were false then 0 is the least element of S. Furthermore, let n be a natural number, and suppose P(m) is true for all natural numbers m less than n + 1. Then if P(n + 1) is false n + 1 is in S, thus being a minimal element in S, a contradiction. Thus P(n + 1) is true.