% BETALN   beta娭悢偺懳悢
% 
% Y = BETALN(Z,W) 偼丄Z 偲 W 偺 懳墳偡傞梫慺偵懳偟偰丄beta娭悢偺帺慠懳悢
% 傪寁嶼偟傑偡丅攝楍 Z 偲W  偼丄摨偠僒僀僘偱側偗傟偽側傝傑偣傫(傑偨偼丄
% 偄偢傟偐偑僗僇儔偱傕峔偄傑偣傫)丅BETALN 偼丄偮偓偺傛偆偵掕媊偝傟傑偡丅
%
%     BETALN = LOG(BETA(Z,W)) 
%
% 幚嵺偵偼丄BETALN 偼 BETA(Z,W) 傪寁嶼偣偢偵摼傜傟傑偡丅beta娭悢偼旕忢偵
% 戝偒偄抣偐傜彫偝偄抣傑偱傪曄摦偱偒傞偺偱丄娭悢偺懳悢偑旕忢偵桳岠偲側傞
% 偙偲偑偁傝傑偡丅
% 
% 嶲峫  BETAINC, BETA.


%   Reference: Abramowitz & Stegun, sec. 6.2.
%   Copyright 1984-2004 The MathWorks, Inc.