>
>I haven't seen the code, but I just want to make you notice that adding together -Infinity and +Infinity results in a NaN ; so does subtracting
>two infinities of the same sign.
>
>So, the NaN might happen when two expressions that are supposed to partially cancel each other, happen to both overflow, in different directions.
>
>There are various possible causes for NaN, but with formulas that only involve +, - and *, the possibilities are a lot more limited.

Hmmm.
I think that's why it puzzles me so. Can you see a / anywhere here?

  while (n--) {
    i1=(*in++);
    fc1 = (*fc++);
    res1 = (*res++);
    q = 1.0f - fc1;
    p = fc1 + 0.8f * fc1 * q;
    fcoeff = p + p - 1.0f;
    q = res1 * (1.0f + 0.5f * q * (1.0f - q + 5.6f * q * q));
    i1 -= q * xb4;
    t1 = xb1;
    xb1 = (i1 + xb0) * p - xb1 * fcoeff;
    t2 = xb2;
    xb2 = (xb1 + t1) * p - xb2 * fcoeff;
    t1 = xb3;
    xb3 = (xb2 + t2) * p - xb3 * fcoeff;
    xb4 = (xb3 + t1) * p - xb4 * fcoeff;
    xb4 = xb4 - xb4 * xb4 * xb4 * 0.166667f;
    xb0 = i1;
    *out++ = xb4;
 }

No reciprocals, no divisions, just +,-,*.
I cannot help thinking I must be reaching a "limit" in the calculus sense...

Ed




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