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The ideal boson gas: Introduction

For the bosonic ideal gas, one must solve the equations

eqnarray57

in order to obtain the equation of state. Examination of these equations, however, shows an immediate problem: The term tex2html_wrap_inline632 is divergent both for the pressure and the average particle number. These terms need to be treated carefully, and so we split them off from the rest of the sum, giving:

eqnarray65

where tex2html_wrap_inline634 means that the tex2html_wrap_inline632 term is excluded. With these divergent terms split off, the thermodynamic limit can be taken and the remaining sums converted to integrals as was done in the fermion case. Thus, for the pressure, we find

eqnarray74

where the change of variables

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has been made. Using the expansion

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the pressure equation becomes

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and by a similar procedure, the average particle number becomes

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In this equation, the term that has been split off represents the average occupation of the ground ( tex2html_wrap_inline632 ) energy state:

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Since tex2html_wrap_inline640 must be greater than or equal to 0, it can be seen that there are restrictions on the allowed values of tex2html_wrap_inline628 . Firstly, since tex2html_wrap_inline644 , tex2html_wrap_inline628 must be a positive number. However, in order that the average occupation of the ground state be positive,

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from which it follows that

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The fact that as tex2html_wrap_inline630 causes tex2html_wrap_inline640 to diverge will have interesting consequences to be discussed below. However, let us first consider the low density limit with tex2html_wrap_inline652 .



Mark Tuckerman
Sat Jan 4 22:50:24 EST 2003