Difference between revisions of "Cardinality of underlying set of a profinite group need not determine order as a profinite group"

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==Statement==
 
==Statement==
  
It is possible to have two [[profinite group]]s <math>G_1</math> and <math>G_2</math> that have the same [[order of a group|order as each other as abstract groups]] but such that the [[order of a profinite group|order as a profinite group]] for <math>G_1</math> is not the same as the order as a profinite group for <math>G_2</math>. Note that the equality of orders as profinite groups is checked in the sense of equality of [[supernatural number]]s. [[fact about::order of a profinite group;2| ]]
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It is possible to have two [[profinite group]]s <math>G_1</math> and <math>G_2</math> that have the same [[order of a group|order as each other as abstract groups]] but such that the [[order of a profinite group|order as a profinite group]] for <math>G_1</math> is not the same as the order as a profinite group for <math>G_2</math>. Note that the equality of orders as profinite groups is checked in the sense of equality of [[supernatural number]]s. [[fact about::order of a profinite group;2| ]][[fact about::order of a group;3| ]]
  
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==Related facts==
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* [[Order of a profinite group need not determine order as a group in the sense of cardinality of underlying set]]
 
==Proof==
 
==Proof==
  

Latest revision as of 01:12, 8 June 2012

Statement

It is possible to have two profinite groups G_1 and G_2 that have the same order as each other as abstract groups but such that the order as a profinite group for G_1 is not the same as the order as a profinite group for G_2. Note that the equality of orders as profinite groups is checked in the sense of equality of supernatural numbers.

Related facts

Proof

Let K_1 be cyclic group:Z2 and K_2 be cyclic group:Z3. Consider the groups G_1 = K_1^\omega and G_2 = K_2^\omega. As a group, G_1 is the countable times unrestricted external direct product of K_1 with itself. The topology is the product topology from the discrete topology of K_1. Similarly, as a group, G_2 is the countable times unrestricted external direct product of K_2 with itself. The topology is the product topology from the discrete topology of K_2.

Then, we note that:

  • The cardinalities of the underlying sets of G_1 and G_2 are equal: Th cardinality of the underlying set of G_1 is 2^\omega, which is the cardinality of the continuum, i.e., the power cardinal of the first infinite ordinal. The cardinality of the underlying set of G_2 is 3^\omega = 2^\omega, which is the cardinality of the continuum, i.e., the power cardinal of the first infinite ordinal. Thus both cardinals are equals.
  • The order (as a profinite group) of G_1 is not equal, as a supernatural number, to the order (as a profinite gorup) of G_2: The order of G_1 is 2^\infty and the order of G_2 is 3^\infty.