Terence Tao was born in 1975

Two amateur mathematicians were at UCLA to hear a lecture delivered by Terence Tao. One of them, Optimist Oliver said to his friend Pessimist Peter.

Oliver : I am going to impress him with my discovery. I have this number p p which has exactly 1975 1975 factors and also has its last 4 4 digits as 1975 \cdots 1975 ; and we all know that Terence Tao was born in 1975 1975 .

Peter : You fool. You have made a mistake.

If you think Peter is right, what could be the correct number of factors of p p . If you think that Oliver is right, choose 1975 1975

1976 1975 1974

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1 solution

Optimistic Oliver must have made a calculation mistake.

For any number, N N with prime factorization, N = p 1 i 1 p 2 i 2 p n i n N=p_1^{i_1}p_2^{i_2}\cdots p_n^{i_n} , the total number of factors can be calculated to be n f = ( i 1 + 1 ) ( i 2 + 1 ) ( i n + 1 ) n_f=(i_1+1)(i_2+1)\cdots(i_n+1) .

Now, analysing the last digits of N N in the problem, we can see that the number ends in 1975 \cdots 1975 .

Now noting that for any a a , 10 a + 5 5 = 2 a + 1 \frac{10a+5}{5}=2a+1 , and any number has to end in 5 5 or 0 0 to be divisible by 5 5 ; we can conclude that N N divides 25 25 , but is not divisible by 125 125 .

N 5 = 395 , N 25 = 79 \frac{N}{5} = \cdots 395, \frac{N}{25} = \cdots 79

From this, we get that N = 5 2 p 2 i 2 p n i n N = 5^2p_2^{i_2}\cdots p_n^{i_n} and therefore the number of factors would be n f = 3 ( i 2 + 1 ) ( i n + 1 ) n_f = 3(i_2+1)\cdots(i_n+1) .

The number of factors should be divisible by 3 3 . 1975 1975 is not divisible by 3 3 . Hence, Oliver is wrong.

Of the other two choices provided, 1974 \boxed{1974} is divisible by 3 3

For the sake of completeness, can you prove that such a number, which has exactly 1974 factors and ends in 1975 in the decimal system really exists?

Zee Ell - 3 years, 6 months ago

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