Quasi-periodic oscillations (QPOs) are sub-second variability observed during outbursts in black
hole low-mass X-ray binaries (BH LMXBs), and their origin is still under debate. The QPO
analysis has basically relied on the fast Fourier transform (FFT) in which the frequency resolution
degrades as the length of the light curve gets shorter. Here we applied the period analysis by
the least absolute shrinkage and selection operator (Lasso) to the X-ray light curve of the BH
LMXB named XTE J1550−564, in which type-C QPOs having the peak frequency of 5.7 Hz
were detected, and compared the FFT and Lasso spectra. The FFT power spectrum becomes
messy around the QPO peak frequency as the length of the light curve is shorter and shorter. On
the other hand, we found a single and sharp peak in the Lasso power spectrum of the 1.4-s light
curve. We repeated estimating the Lasso spectrum of each 1.4-s light curve by shifting the window
throughout the 2-ks entire light curve, and obtained the half width at half maximum (HWHM) of
the histogram of the resolved QPO frequencies. The HWHM was consistent with the HWHM of
the Lorentzian in the FFT spectrum of the entire light curve. These results suggest that the Lasso
technique is powerful for investigating the short-term variability of the QPO frequency and that
the Lasso analysis result is consistent with the FFT analysis result.