Nice number

A number is said to be a nice number if it has exactly 4 factors (Including one and number itself). let n = 2 3 × 3 2 × 5 3 × 7 × 11 2 n={ 2 }^{ 3 }\times { 3 }^{ 2 }\times { 5 }^{ 3 }\times 7\times { 11 }^{ 2 } then number of factors. How many nice numbers are there?


The answer is 12.

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1 solution

Aditya Raut
Jul 13, 2014

We know that if a number is of the form

n = p 1 a 1 × p 2 a 2 × p 3 a 3 . . . . × p n a n n = p_1^{a_1}\times p_2^{a_2} \times p_3^{a_3} .... \times p_n^{a_n} ... where a i a_i s are primes,

then the number of it's factors is ( a 1 + 1 ) ( a 2 + 1 ) ( a 3 + 1 ) . . . ( a n + 1 ) (a_1+1)(a_2+1)(a_3+1)...(a_n+1) ... this is because in the factor, the power of prime p j p_j could be anything from 0 0 to a j a_j implying j + 1 j+1 different powers and talking about all primes together will give that number of factors.

Thus if you want a number to have 4 factors exactly, it HAS to be of the form p 3 p^3 where p p is prime, or of the form p 1 × p 2 p_1\times p_2 where p 1 p_1 and p 2 p_2 are primes.

Here you have 5 choices of primes, and they are 2 , 3 , 5 , 7 , 11 2,3,5,7,11 so you have to chose 2 out of them and have power 1. So there are ( 5 2 ) = 10 \dbinom{5}{2} =10 numbers.

And other way, the cubes can only be 2 3 2^3 and 5 3 5^3 , so there will be 2 more numbers.

That accounts to give total 12 \boxed{12} numbers.

Did it the same way !!

CH Nikhil - 6 years, 6 months ago

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