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* double a, b, x, y, btdtr();
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* y = btdtr( a, b, x );
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* Returns the area from zero to x under the beta density
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* P(x) = ---------- | t (1-t) dt
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* This function is identical to the incomplete beta
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* integral function incbet(a, b, x).
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* The complemented function is
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* 1 - P(1-x) = incbet( b, a, x );
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Cephes Math Library Release 2.0: April, 1987
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Copyright 1984, 1987, 1995 by Stephen L. Moshier
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extern double incbet ( double aa, double bb, double xx );
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double btdtr( double,double,double );
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double btdtr( a, b, x )
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return( incbet( a, b, x ) );