## Wednesday, June 13, 2007

### Who invented the Equal Sign?

A bit of math trivia for you today. The equals sign = was invented in 1557 by Welsh mathematician Robert Recorde. In his book, The Whetstone of Witte, he wrote, "To avoid the tedious repetition of these words: is equal to; I will set a pair of parallel lines thus, =, because no two things can be more equal.

Here's your question: Perfect numbers are special because they are equal to the sum of their factors. The number 6, for instance, has factors 1, 2, and 3 which when summed equals 6. The next perfect number is 28. Do you know what the third perfect number is?

Hint: It's less than 500.

1. If memory serves:

Euclid's formula
2^(n-1)*(2^n - 1)

where n is a prime number:

n=2 > 6
n=3 > 28
n=5 > 496

2. very impressive, Mr. Don. Keep this up and I'll start calling you Dr. Don, or Professor Don.

3. Looks like you remembered correctly. The answer is 496. Very impressive memory.

4. Wow, very nice formula. I was just going to go to a site and get the first eight perfect numbers.

5. 496

6. 378

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