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In base 12, a square number can end only with square digits (like in base 12, a prime number can end only with prime digits or 1), that is, 0, 1, 4 or 9, as follows: if a number is divisible both by 2 and by 3 (that is, divisible by 6), its square ends in 0, and its preceding digit must be 0 or 3;
The final digit of a triangular number is 0, 1, 3, 5, 6, or 8, and thus such numbers never end in 2, 4, 7, or 9. A final 3 must be preceded by a 0 or 5; a final 8 must be preceded by a 2 or 7. In base 10, the digital root of a nonzero triangular number is always 1, 3, 6, or 9. Hence, every triangular number is either divisible by three or has a ...
where each e i is either 0 or 1. There are 2 k ways of forming the square-free part of a. And s 2 can be at most N, so s ≤ √ N. Thus, at most 2 k √ N numbers can be written in this form. In other words, . Or, rearranging, k, the number of primes less than or equal to N, is greater than or equal to 1 / 2 log 2 N.
Some numbers, like 36, can be arranged both as a square and as a triangle (see square triangular number): By convention, 1 is the first polygonal number for any number of sides. The rule for enlarging the polygon to the next size is to extend two adjacent arms by one point and to then add the required extra sides between those points.
Palindromic number. A palindromic number (also known as a numeral palindrome or a numeric palindrome) is a number (such as 16461) that remains the same when its digits are reversed. In other words, it has reflectional symmetry across a vertical axis. The term palindromic is derived from palindrome, which refers to a word (such as rotor or ...
The sequence of highly composite numbers (sequence A002182 in the OEIS) is a subset of the sequence of smallest numbers k with exactly n divisors (sequence A005179 in the OEIS ). Highly composite numbers whose number of divisors is also a highly composite number are. 1, 2, 6, 12, 60, 360, 1260, 2520, 5040, 55440, 277200, 720720, 3603600 ...
All telephone numbers in Iran have 11 digits (initial 0 and ten digits). The first two or three digits after the zero are the area code. The possibilities are: (0xx) xxxx xxxx (for landlines), 09xx xxx xxxx (for cellphones) and 099xx xxx xxx (for MVNO ). When making a call within the same landline area code, initial 0 plus the area code must be ...
Carmichael number. In number theory, a Carmichael number is a composite number which in modular arithmetic satisfies the congruence relation : for all integers . [1] The relation may also be expressed [2] in the form: for all integers that are relatively prime to . They are infinite in number.