Eighty One (81)
1. It is a square number.
9^2 = 81
2. It is the sum of first nine odd integers.
1 + 3 + 5 + 7 + 9 + 11 + 13 + 15 + 17 = 81
3. It is the square of sum of its digits. Apart from 0 and 1 it is the only number with this property.
81 = ( 8 + 1)^2
4. It is the fourth power of a number.
3^4 = 81
5. It is the smallest square whose sum of divisors is also a square number. The divisors of 81 are 1, 3, 9, 27 and 81 and their sum is 121 which is a square number.
1 + 3 + 9 + 27 + 81= 121 = 11^2
6. It is the sum of 9 consecutive numbers.
5 + 6 + 7 + 8 + 9 + 10 + 11 + 12 + 13 = 81
7. The fraction 1/ 81= 0.01245679 012345679… is a repeating decimal containing every digits from 0 to 9 except 8.
8. On base 10, it is a Harshad number.
9. It is a heptagonal number as well as centered octagonal number.
10. The atomic number of Thallium is 81.
Dr Rajesh Kumar Thakur
rkthakur1974@gmail.com
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