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An important special case is when the index set is , the natural numbers: this Cartesian product is the set of all infinite sequences with the i-th term in its corresponding set X i. For example, each element of ∏ n = 1 ∞ R = R × R × ⋯ {\displaystyle \prod _{n=1}^{\infty }\mathbb {R} =\mathbb {R} \times \mathbb {R} \times \cdots } can ...
Using first-order logic primitive symbols, the axiom can be expressed as follows: [2] ( ( ()) ( ( (( =))))). In English, this sentence means: "there exists a set 𝐈 such that the empty set is an element of it, and for every element of 𝐈, there exists an element of 𝐈 such that is an element of , the elements of are also elements of , and nothing else is an element of ."
Cantor's diagonal argument (among various similar names [note 1]) is a mathematical proof that there are infinite sets which cannot be put into one-to-one correspondence with the infinite set of natural numbers – informally, that there are sets which in some sense contain more elements than there are positive integers.
The aleph numbers differ from the infinity commonly found in algebra and calculus, in that the alephs measure the sizes of sets, while infinity is commonly defined either as an extreme limit of the real number line (applied to a function or sequence that "diverges to infinity" or "increases without bound"), or as an extreme point of the ...
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.
Some sources allow convergence to 0 if there are only a finite number of zero factors and the product of the non-zero factors is non-zero, but for simplicity we will not allow that here. If the product converges, then the limit of the sequence a n as n increases without bound must be 1, while the converse is in general not true.
A morphism φ : (X, 0 X, S X) → (Y, 0 Y, S Y) is a C-morphism φ : X → Y with φ 0 X = 0 Y and φ S X = S Y φ. Then C is said to satisfy the Dedekind–Peano axioms if US 1 ( C ) has an initial object; this initial object is known as a natural number object in C .
The logarithm of an infinite cardinal number κ is defined as the least cardinal number μ such that κ ≤ 2 μ. Logarithms of infinite cardinals are useful in some fields of mathematics, for example in the study of cardinal invariants of topological spaces, though they lack some of the properties that logarithms of positive real numbers possess.