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144, or twice 72, is also highly totient, as is 576, the square of 24. [8] While 17 different integers have a totient value of 72, the sum of Euler's totient function φ(x) over the first 15 integers is 72. [9] It also is a perfect indexed Harshad number in decimal (twenty-eighth), as it is divisible by the sum of its digits . [10]
The basic rule for divisibility by 4 is that if the number formed by the last two digits in a number is divisible by 4, the original number is divisible by 4; [2] [3] this is because 100 is divisible by 4 and so adding hundreds, thousands, etc. is simply adding another number that is divisible by 4. If any number ends in a two digit number that ...
The natural log of 2 is 0.693147, so when you solve for t using those natural logarithms, you get t = 0.693147/r.. The actual results aren’t round numbers and are closer to 69.3, but 72 easily ...
The choice of number is mostly a matter of preference: 69 is more accurate for continuous compounding, while 72 works well in common interest situations and is more easily divisible. There are a number of variations to the rules that improve accuracy. For periodic compounding, the exact doubling time for an interest rate of r percent per period is
d() is the number of positive divisors of n, including 1 and n itself; σ() is the sum of the positive divisors of n, including 1 and n itselfs() is the sum of the proper divisors of n, including 1 but not n itself; that is, s(n) = σ(n) − n
360 is divisible by the number of its divisors , and it is the smallest number divisible by every natural number from 1 to 10, except 7. Furthermore, one of the divisors of 360 is 72, which is the number of primes below it. 360 is the sum of twin primes (179 + 181) and the sum of four consecutive powers of three (9 + 27 + 81 + 243).
For example, 72 = 2 3 × 3 2, all the prime factors are repeated, so 72 is a powerful number. 42 = 2 × 3 × 7, none of the prime factors are repeated, so 42 is squarefree. Euler diagram of numbers under 100:
The year has to be evenly divisible by 4 to be a leap year. It’s not a leap year if the year can be evenly divided by 100 unless that year can also be evenly divided by 400, thanks to Pope ...