Function BESSI(N, X)
    IACC = 40
    BIGNO = 10000000000#
    BIGNI = 0.0000000001
    If N < 2 Then Print "bad argument N in BESSI"
    TOX = 2# / X
    BIP = 0#
    BI = 1#
    BESSI = 0#
    M = 2 * ((N + Int(Sqr(IACC * N))))
    For J = M To 1 Step -1
        BIM = BIP + J * TOX * BI
        BIP = BI
        BI = BIM
        If Abs(BI) > BIGNO Then
            BESSI = BESSI * BIGNI
            BI = BI * BIGNI
            BIP = BIP * BIGNI
        End If
        If J = N Then BESSI = BIP
    Next J
    BESSI = BESSI * BESSI0(X) / BI
End Function