In Problems 19 through 28, investigate the type of the critical point (0 , 0) of the given almost linear system. Verify your conclusion by using a computer system or graphing calculator to construct a phase portrait. Also, describe the approximate locations and apparent types of any other critical points that are visible in your figure. Feel free to investigate these additional critical points; you can use the computational methods discussed in the application material for this section.

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dx di = x - y + x4 - y2, dt dy dt = 2xy + y4 - x²