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  2. Direct product of groups - Wikipedia

    en.wikipedia.org/wiki/Direct_product_of_groups

    The resulting algebraic object satisfies the axioms for a group. Specifically: Associativity The binary operation on G × H is associative. Identity The direct product has an identity element, namely (1 G, 1 H), where 1 G is the identity element of G and 1 H is the identity element of H.

  3. Perfect number - Wikipedia

    en.wikipedia.org/wiki/Perfect_number

    The sum of proper divisors of a number is called its aliquot sum, so a perfect number is one that is equal to its aliquot sum. Equivalently, a perfect number is a number that is half the sum of all of its positive divisors; in symbols, σ 1 ( n ) = 2 n {\displaystyle \sigma _{1}(n)=2n} where σ 1 {\displaystyle \sigma _{1}} is the sum-of ...

  4. Riemann sum - Wikipedia

    en.wikipedia.org/wiki/Riemann_sum

    Specific choices of give different types of Riemann sums: . If = for all i, the method is the left rule [2] [3] and gives a left Riemann sum.; If = for all i, the method is the right rule [2] [3] and gives a right Riemann sum.

  5. Sum rule in quantum mechanics - Wikipedia

    en.wikipedia.org/wiki/Sum_rule_in_quantum_mechanics

    In quantum mechanics, a sum rule is a formula for transitions between energy levels, in which the sum of the transition strengths is expressed in a simple form. Sum rules are used to describe the properties of many physical systems, including solids, atoms, atomic nuclei, and nuclear constituents such as protons and neutrons.

  6. Formal sum - Wikipedia

    en.wikipedia.org/wiki/Formal_sum

    In linear algebra, an element of a vector space, a sum of finitely many elements from a given basis set multiplied by real, complex, or other numerical coefficients. In the study of series (mathematics) , a sum of an infinite sequence of numbers or other quantities, considered as an abstract mathematical object regardless of whether the sum ...

  7. Lydia Shum - Wikipedia

    en.wikipedia.org/wiki/Lydia_Shum

    Lydia Shum Din-ha or Lydia Tin Ha Sum (Chinese: 沈殿霞; 21 July 1945 – 19 February 2008) was a Hong Kong-Canadian comedian, MC, actress and singer. Known for her portly figure, signature dark-rimmed glasses and bouffant hairstyle, she was affectionately known to peers and fans as Fei-fei ( Chinese : 肥肥 , lit.