Ash (1998) includes the following areas in his definition of abstract algebra: logic and foundations, counting, elementary number theory, informal set theory, linear algebra, and the theory of linear operators.
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The spring equinox always occurs on March 20/21/22 Gregorian in every year. But not in the Julian calendar, which mankind used before 1582, because it does not track the solar year properly. It is a bit short because it makes every year 365.25 days long whereas in fact they are 365.2421896698 days long. The first true day of spring is the first full day after the vernal equinox.