数学算法

				Function BESSI(N:integer; X:real):real;
				const
				    IACC = 40;    BIGNO = 1.e10;
				    BIGNI = 1.e-10;
				var
				    TOX,BIP,BI,BIM,ANS:real; M,J:integer;
				begin
				    If N < 2 Then
				        ShowMessage('bad argument N in BESSI');
				    TOX:=2 / X;
				    BIP:=0;
				    BI:=1;
				    BESSI:=0;
				    M:=2 * ((N + Trunc(Sqrt(IACC * N))));
				    For J:=M DownTo 1 do
				    begin
				        BIM:=BIP + J * TOX * BI;
				        BIP:=BI;
				        BI:=BIM;
				        If Abs(BI) > BIGNO Then
				        begin
				            ANS:=ANS * BIGNI;
				            BI:=BI * BIGNI;
				            BIP:=BIP * BIGNI;
				        end;
				        If J = N Then ANS:=BIP;
				    end;
				    BESSI:=ANS * BESSI0(X) / BI;
				end;