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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 digit sum of 2946, for example is 2 + 9 + 4 + 6 = 21. Since 21 = 2946 − 325 × 9, the effect of taking the digit sum of 2946 is to "cast out" 325 lots of 9 from it. If the digit 9 is ignored when summing the digits, the effect is to "cast out" one more 9 to give the result 12. More generally, when casting out nines by summing digits, any ...
4 9 8 48: 4,1 5 10 9 60* 2,1,1 ... The first highly composite number that is not a Harshad number is 245,044,800; it has a digit sum of 27, which does not divide ...
Plot of the number of divisors of integers from 1 to 1000. Highly composite numbers are in bold and superior highly composite numbers are starred. ... 4, 6, 9, 12, 18 ...
It is a square number (33 squared), a nonagonal number, [1] a 32-gonal number, a 364-gonal number, and a centered octagonal number. [2] 1089 is the first reverse-divisible number. The next is 2178 (= 1089 × 2 = 8712/4), and they are the only four-digit numbers that divide their reverse.
The number 18 is a harshad number in base 10, because the sum of the digits 1 and 8 is 9, and 18 is divisible by 9.; The Hardy–Ramanujan number (1729) is a harshad number in base 10, since it is divisible by 19, the sum of its digits (1729 = 19 × 91).
if a number is divisible neither by 2 nor by 3, its square ends in 1, and its preceding digit must be even; if a number is divisible by 2, but not by 3, its square ends in 4, and its preceding digit must be 0, 1, 4, 5, 8, or 9; and; if a number is not divisible by 2, but by 3, its square ends in 9, and its preceding digit must be 0 or 6.
In number theory, reversing the digits of a number n sometimes produces another number m that is divisible by n. This happens trivially when n is a palindromic number; the nontrivial reverse divisors are 1089, 2178, 10989, 21978, 109989, 219978, 1099989, 2199978, ... (sequence A008919 in the OEIS).