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| {{Distinguish|Number}}
| | Hello. Let me introduce the author. Her title is Refugia Shryock. Hiring has been my occupation for some time but I've already applied for another 1. Minnesota is where he's been residing for many years. Doing ceramics is what my family members and I appreciate.<br><br>Here is my website ... [http://sinchonart.com/?mid=board1&sort_index=regdate&order_type=desc&document_srl=677856&trackback_srl=99909 at home std testing] |
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| In [[mathematics]], the '''nimbers''', also called '''Grundy numbers''', are introduced in [[combinatorial game theory]], where they are defined as the values of [[nim]] heaps. They arise in a much larger class of games because of the [[Sprague–Grundy theorem]]. The nimbers are the [[ordinal number]]s endowed with a new '''nimber addition''' and '''nimber multiplication''', which are distinct from ordinal addition and ordinal multiplication.
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| ==Properties==
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| The [[Sprague–Grundy theorem]] states that every [[impartial game]] is equivalent to a nim heap of a certain size. Nimber addition (also known as '''nim-addition''') can be used to calculate the size of a single heap equivalent to a collection of heaps. It is defined recursively by
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| :<math>\alpha + \beta = \operatorname{mex}(\{\,\alpha' + \beta : \alpha' < \alpha\,\} \cup \{\, \alpha + \beta' : \beta' < \beta \,\}),</math>
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| where for a [[Set (mathematics)|set]] ''S'' of ordinals, [[mex (mathematics)|mex]](''S'') is defined to be the "minimum excluded ordinal", i.e. mex(''S'') is the smallest ordinal which is not an element of ''S''.
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| ===Addition===
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| For finite ordinals, the '''nim-sum''' is easily evaluated on a computer by taking the [[Bitwise operation|bitwise]] [[exclusive or]] (XOR, denoted by ⊕) of the corresponding numbers. It can also be evaluated by hand by adding the binary representations of the corresponding numbers and treating even digits as 0. For example, the nim-sum of 7 and 14 can be found by writing 7 as 111 and 14 as 1110; the ones place adds to 1; the twos place adds to 2, which we replace with 0; the fours place adds to 2, which we replace with 0; the eights place adds to 1. So the nim-sum is written in binary as 1001, or in decimal as 9.
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| This property of addition follows from the fact that both mex and XOR yield a winning strategy for Nim and there can be only one such strategy; or it can be shown directly by induction: Let α and β be two finite ordinals, and assume that the nim-sum of all pairs with one of them reduced is already defined. The only number whose XOR with α is α ⊕ β is β, and vice versa; thus α ⊕ β is excluded. On the other hand, for any ordinal γ < α ⊕ β, XORing ξ := α ⊕ β ⊕ γ with all of α, β and γ must lead to a reduction for one of them (since the leading 1 in ξ must be present in at least one of the three); since ξ ⊕ γ = α ⊕ β > γ, we must have α > ξ ⊕ α = β ⊕ γ or β > ξ ⊕ β = α ⊕ γ; thus γ is included as (β ⊕ γ) ⊕ β or as α ⊕ (α ⊕ γ), and hence α ⊕ β is the minimum excluded ordinal.
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| ===Multiplication===
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| Nimber multiplication ('''nim-multiplication''') is defined recursively by
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| :α β = mex{α ′ β + α β ′ − α ′ β ′ : α ′ < α, β ′ < β} = mex{α ′ β + α β ′ + α ′ β ′ : α ′ < α, β ′ < β}.
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| Except for the fact that nimbers form a [[class (set theory)|proper class]] and not a set, the class of nimbers determines an [[algebraically closed field]] of [[characteristic (algebra)|characteristic]] 2. The nimber additive identity is the ordinal 0, and the nimber multiplicative identity is the ordinal 1. In keeping with the characteristic being 2, the nimber additive inverse of the ordinal α is α itself. The nimber multiplicative inverse of the nonzero ordinal α is given by 1/α = mex(''S''), where ''S'' is the smallest set of ordinals (nimbers) such that
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| # 0 is an element of ''S'';
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| # if 0 < α ′ < α and β ′ is an element of ''S'', then [1 + (α ′ − α) β ′ ]/α ′ is also an element of ''S''.
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| For all natural numbers ''n'', the set of nimbers less than 2<sup>2<sup>''n''</sup></sup> form the [[Galois field]] GF(2<sup>2<sup>''n''</sup></sup>) of order 2<sup>2<sup>''n''</sup></sup>.
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| In particular, this implies that the set of finite nimbers is isomorphic to the [[direct limit]] of the fields GF(2<sup>2<sup>''n''</sup></sup>), for each positive n. This subfield is not algebraically closed, however. {{Cn|date=September 2012}}
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| Just as in the case of nimber addition, there is a means of computing the nimber product of finite ordinals. This is determined by the rules that
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| # The nimber product of distinct Fermat 2-powers (numbers of the form 2<sup>2<sup>''n''</sup></sup>) is equal to their ordinary product;
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| # The nimber square of a Fermat 2-power ''x'' is equal to 3''x''/2 as evaluated under the ordinary multiplication of natural numbers.
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| The smallest algebraically closed field of nimbers is the set of nimbers less than the ordinal ''ω''<sup>''ω''<sup>''ω''</sup></sup>, where ''ω'' is the smallest infinite ordinal. It follows that as a nimber, ''ω''<sup>''ω''<sup>''ω''</sup></sup> is [[transcendental number|transcendental]] over the field.{{Cn|date=September 2012}}
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| == Addition and multiplication tables ==
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| The following tables exhibit addition and multiplication among the first 16 nimbers. <br>
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| This subset is closed under both operations, since 16 is of the form 2<sup>2<sup>''n''</sup></sup>
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| <small>(If you prefer simple text tables, they are [http://en.wikipedia.org/w/index.php?title=Nimber&oldid=383699838#Addition_and_multiplication_tables here].)</small>
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| [[File:Z2^4; Cayley table; binary.svg|thumb|center|500px|Nimber addition {{OEIS|A003987}}<br>This is also the [[Cayley table]] of [[List_of_small_groups#List_of_small_abelian_groups|Z<sub>2</sub><sup>4</sup>]] - or the table of [[Bitwise operation|bitwise]] [[w:Exclusive or|XOR]] operations.<br>The small matrices show the single digits of the binary numbers.]]
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| [[File:Nimbers 0...15 multiplication.svg|thumb|center|500px|Nimber multiplication {{OEIS|A051775}}<br>The nonzero elements form the [[w:Cayley table|Cayley table]] of [[w:List_of_small_groups#List_of_small_abelian_groups|Z<sub>15</sub>]].<br>The small matrices are permuted binary [[Walsh matrix|Walsh matrices]].]]
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| [[File:Nimber products of powers of two.svg|thumb|center|900px|Nimber multiplication of [[Power of two|powers of two]] {{OEIS|A223541}}<br>Calculating the nim-products of powers of two is a decisive point in the recursive algorithm of nimber-multiplication.]]
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| ==References== | |
| {{refbegin}}
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| * {{cite book
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| |first1=John Horton
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| |last1=Conway
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| |authorlink1=John Horton Conway
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| |title=[[On Numbers and Games]]
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| |publisher=[[Academic Press]] Inc. (London) Ltd.
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| |year=1976
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| }}
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| * {{cite web
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| |first1=H. W.
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| |last1=Lenstra
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| |authorlink1=Hendrik Willem Lenstra, Jr.
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| |title=Nim multiplication
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| |id={{hdl|1887/2125}}
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| |year=1978
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| }}
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| * {{cite arxiv
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| |first1=Dierk
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| |last1=Schleicher
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| |first2=Michael
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| |last2=Stoll
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| |eprint=math.DO/0410026
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| |title= An Introduction to Conway's Games and Numbers}} which discusses games, [[surreal number]]s, and nimbers.
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| {{refend}}
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| [[Category:Combinatorial game theory]]
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| [[Category:Finite fields]]
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Hello. Let me introduce the author. Her title is Refugia Shryock. Hiring has been my occupation for some time but I've already applied for another 1. Minnesota is where he's been residing for many years. Doing ceramics is what my family members and I appreciate.
Here is my website ... at home std testing