All examples in this Problem are groups of black dots.
The nth triangular number is the sum over the natural numbers from 1 to n, where n > 0. Note: 0 is the 0th triangular number. The first few triangular numbers are 0, 1, 3 (= 1+2) and 6 (= 1+2+3)
Adjacent-numbered pages: BP564 BP565 BP566 BP567 BP568  *  BP570 BP571 BP572 BP573 BP574
nice, precise, allsorted, notso, number, math, left-narrow, left-null, help, preciseworld
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Leo Crabbe