That minus 81, it’s not in this progression. The terms are getting smaller by 3, so they'll all stay positive until they hit zero. Zero *is* a term—it's the eighth one. This pension paperwork of mine, see, it taught me to check the sequence properly, line by line, because what you expect isn't always there. So I sat on the porch and worked it out slowly: 21, 18, 15... down to zero, but never a negative like minus 81.
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