unit Unit2;

interface
Function BESSI0(X:REAL):REAL;

implementation
Function BESSI0(X:REAL):REAL;
const
    P1 = 1;             P2 = 3.5156229;
    P3 = 3.0899424;     P4 = 1.2067492;
    P5 = 0.2659732;     P6 = 0.0360768;
    P7 = 0.0045813;
    Q1 = 0.39894228;    Q2 = 0.01328592;
    Q3 = 0.00225319;    Q4 = -0.00157565;
    Q5 = 0.00916281;    Q6 = -0.02057706;
    Q7 = 0.02635537;    Q8 = -0.01647633;
    Q9 = 0.00392377;
var
    Y,AAA,BBB,AX:Real;
begin
    If Abs(X) < 3.75 Then
    begin
        Y:=Sqr(X / 3.75);
        AAA:=Y * (P5 + Y * (P6 + Y * P7));
        BESSI0:=P1 + Y * (P2 + Y * (P3 + Y * (P4 + AAA)));
    end
    Else
    begin
        AX:=Abs(X);
        Y:=3.75 / AX;
        AAA:=Exp(AX) / Sqrt(AX);
        BBB:=Q4 + Y * (Q5 + Y * (Q6 + Y * (Q7 + Y * (Q8 + Y * Q9))));
        BESSI0:=AAA * (Q1 + Y * (Q2 + Y * (Q3 + Y * BBB)));
    end;
end;

end.