commons-math-legacy/src/main/java/org/apache/commons/math4/legacy/ode/nonstiff/EmbeddedRungeKuttaIntegrator.java [260:279]:
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        for (int k = 1; k < stages; ++k) {

          for (int j = 0; j < y0.length; ++j) {
            double sum = a[k-1][0] * yDotK[0][j];
            for (int l = 1; l < k; ++l) {
              sum += a[k-1][l] * yDotK[l][j];
            }
            yTmp[j] = y[j] + stepSize * sum;
          }

          computeDerivatives(stepStart + c[k-1] * stepSize, yTmp, yDotK[k]);
        }

        // estimate the state at the end of the step
        for (int j = 0; j < y0.length; ++j) {
          double sum    = b[0] * yDotK[0][j];
          for (int l = 1; l < stages; ++l) {
            sum    += b[l] * yDotK[l][j];
          }
          yTmp[j] = y[j] + stepSize * sum;
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commons-math-legacy/src/main/java/org/apache/commons/math4/legacy/ode/nonstiff/RungeKuttaIntegrator.java [146:165]:
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      for (int k = 1; k < stages; ++k) {

          for (int j = 0; j < y0.length; ++j) {
              double sum = a[k-1][0] * yDotK[0][j];
              for (int l = 1; l < k; ++l) {
                  sum += a[k-1][l] * yDotK[l][j];
              }
              yTmp[j] = y[j] + stepSize * sum;
          }

          computeDerivatives(stepStart + c[k-1] * stepSize, yTmp, yDotK[k]);
      }

      // estimate the state at the end of the step
      for (int j = 0; j < y0.length; ++j) {
          double sum    = b[0] * yDotK[0][j];
          for (int l = 1; l < stages; ++l) {
              sum    += b[l] * yDotK[l][j];
          }
          yTmp[j] = y[j] + stepSize * sum;
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