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Subtract 9 times the last digit from the rest. The result must be divisible by 13. (Works because 91 is divisible by 13). 637: 63 − 7 × 9 = 0. 14: It is divisible by 2 and by 7. [6] 224: it is divisible by 2 and by 7. Add the last two digits to twice the rest. The result must be divisible by 14. 364: 3 × 2 + 64 = 70, 1,764: 17 × 2 + 64 ...
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
In the past 500 years, there was no leap day in 1700, 1800 and 1900, but 2000 had one. In the next 500 years, if the practice is followed, there will be no leap day in 2100, 2200, 2300 and 2500 ...
For example, 96 = 2 5 × 3 satisfies the above conditions and has 12 divisors but is not highly composite since there is a smaller number (60) which has the same number of divisors. Asymptotic growth and density
A year divisible by 100 is not a leap year in the Gregorian calendar unless it is also divisible by 400. For example, 1600 was a leap year, but 1700, 1800 and 1900 were not. Some programs may have relied on the oversimplified rule that "a year divisible by four is a leap year".
In 1968 Martin Gardner noted that most even amicable pairs sumsdivisible by 9, [12] and that a rule for characterizing the exceptions (sequence A291550 in the OEIS) was obtained. [ 13 ] According to the sum of amicable pairs conjecture, as the number of the amicable numbers approaches infinity, the percentage of the sums of the amicable pairs ...
A century leap year is a leap year in the Gregorian calendar that is evenly divisible by 400. [1] Like all leap years, it has an extra day in February for a total of 366 days instead of 365. In the obsolete Julian calendar, all years that were divisible by 4, including end-of-century years, were considered leap years. The Julian rule, however ...
Arrange the digits 1 to 9 in order so that the first two digits form a multiple of 2, the first three digits form a multiple of 3, the first four digits form a multiple of 4 etc. and finally the entire number is a multiple of 9.