diff --git a/include/SPTK/conversion/linear_predictive_coefficients_to_line_spectral_pairs.h b/include/SPTK/conversion/linear_predictive_coefficients_to_line_spectral_pairs.h
index 2098f66..e268f23 100644
--- a/include/SPTK/conversion/linear_predictive_coefficients_to_line_spectral_pairs.h
+++ b/include/SPTK/conversion/linear_predictive_coefficients_to_line_spectral_pairs.h
@@ -68,7 +68,7 @@ class LinearPredictiveCoefficientsToLineSpectralPairs {
 
   /**
    * @param[in] num_order Order of coefficients, @f$M@f$.
-   * @param[in] num_split Number of splits of unit circle.
+   * @param[in] num_split Number of splits of unit circle quadrant.
    * @param[in] num_iteration Number of iterations.
    * @param[in] convergence_threshold Convergence threshold.
    */
diff --git a/src/conversion/linear_predictive_coefficients_to_line_spectral_pairs.cc b/src/conversion/linear_predictive_coefficients_to_line_spectral_pairs.cc
index 99f4ac8..14c6730 100644
--- a/src/conversion/linear_predictive_coefficients_to_line_spectral_pairs.cc
+++ b/src/conversion/linear_predictive_coefficients_to_line_spectral_pairs.cc
@@ -51,7 +51,7 @@ LinearPredictiveCoefficientsToLineSpectralPairs::
       num_iteration_(num_iteration),
       convergence_threshold_(convergence_threshold),
       is_valid_(true) {
-  if (num_order_ < 0 || num_split_ <= 0 || num_iteration_ <= 0 ||
+  if (num_order_ < 0 || num_split_ <= 0 || num_iteration_ < 0 ||
       convergence_threshold_ < 0.0) {
     is_valid_ = false;
     return;
@@ -123,9 +123,7 @@ bool LinearPredictiveCoefficientsToLineSpectralPairs::Run(
 
   // Search roots of polynomials.
   const double delta(1.0 / num_split_);
-  const double x_max(1.0 - delta);
-  const double x_min(-1.0 - delta);
-  for (double x(x_max); x_min < x; x -= delta) {
+  for (double x(1.0 - delta); -1.0 <= x; x -= delta) {
     double y(CalculateChebyshevPolynomial(*c, x));
 
     if (y * y_prev <= 0.0) {
diff --git a/src/main/lpc2lsp.cc b/src/main/lpc2lsp.cc
index d93d1af..e168ade 100644
--- a/src/main/lpc2lsp.cc
+++ b/src/main/lpc2lsp.cc
@@ -73,7 +73,7 @@ void PrintUsage(std::ostream* stream) {
   *stream << "       -h    : print this message" << std::endl;
   *stream << "     (level 2)" << std::endl;
   *stream << "       -n n  : number of splits of unit circle         (   int)[" << std::setw(5) << std::right << kDefaultNumSplit             << "][   1 <= n <=   ]" << std::endl;  // NOLINT
-  *stream << "       -i i  : maximum number of iterations            (   int)[" << std::setw(5) << std::right << kDefaultNumIteration         << "][   1 <= i <=   ]" << std::endl;  // NOLINT
+  *stream << "       -i i  : maximum number of iterations            (   int)[" << std::setw(5) << std::right << kDefaultNumIteration         << "][   0 <= i <=   ]" << std::endl;  // NOLINT
   *stream << "       -d d  : convergence threshold                   (double)[" << std::setw(5) << std::right << kDefaultConvergenceThreshold << "][ 0.0 <= d <=   ]" << std::endl;  // NOLINT
   *stream << "  infile:" << std::endl;
   *stream << "       linear predictive coefficients                  (double)[stdin]" << std::endl;  // NOLINT
@@ -106,9 +106,9 @@ void PrintUsage(std::ostream* stream) {
  *     \arg @c 2 frequency in kHz
  *     \arg @c 3 frequency in Hz
  * - @b -n @e int
- *   - number of splits of unit circle @f$(1 \le S)@f$
+ *   - number of splits of unit circle quadrant @f$(1 \le S)@f$
  * - @b -i @e int
- *   - maximum number of iterations @f$(1 \le N)@f$
+ *   - maximum number of iterations @f$(0 \le N)@f$
  * - @b -d @e double
  *   - convergence threshold @f$(0 \le \epsilon)@f$
  * - @b infile @e str
@@ -206,10 +206,10 @@ int main(int argc, char* argv[]) {
       }
       case 'i': {
         if (!sptk::ConvertStringToInteger(optarg, &num_iteration) ||
-            num_iteration <= 0) {
+            num_iteration < 0) {
           std::ostringstream error_message;
-          error_message
-              << "The argument for the -i option must be a positive integer";
+          error_message << "The argument for the -i option must be a "
+                        << "non-negative integer";
           sptk::PrintErrorMessage("lpc2lsp", error_message);
           return 1;
         }
diff --git a/test/test_lpc2lsp.bats b/test/test_lpc2lsp.bats
index 8e3c4e0..d1ffaa3 100755
--- a/test/test_lpc2lsp.bats
+++ b/test/test_lpc2lsp.bats
@@ -52,7 +52,7 @@ teardown() {
 
 @test "lpc2lsp: reversibility" {
     $sptk3/nrand -l 400 | $sptk3/lpc -l 400 -m 12 > $tmp/1
-    $sptk4/lpc2lsp -m 12 $tmp/1 | $sptk4/lsp2lpc -m 12 > $tmp/2
+    $sptk4/lpc2lsp -m 12 -i 4 $tmp/1 | $sptk4/lsp2lpc -m 12 > $tmp/2
     run $sptk4/aeq $tmp/1 $tmp/2
     [ "$status" -eq 0 ]
 }