 81 number - In mathematics: Encyclopedia II - 81 number - In mathematicsEighty-one is the square of 9. It is a heptagonal number and a centered octagonal number. It is also a tribonacci number, an open meandric number and a Harshad number.
81 is also one of three numbers (the other two are 1458 and 1729) which, when its digits are added together, produces a sum which, when multiplied by its reversed self, yields the original number:
1 + 8 = 9
9 ยท 9 = 81
(although this case is somewhat dege ...
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