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### 400-Digit Product of Two Primes

```Date: 11/04/2002 at 15:24:38
From: Scott Slovak
Subject: Prime numbers

Hello,

My question is: What two prime numbers when multiplied together will
equal a 400-digit number?

I've tried researching, and I don't even know where to begin.

Thank you very much,
Scott
```

```
Date: 11/05/2002 at 15:20:32
From: Doctor Wilkinson
Subject: Re: Prime numbers

Hi, Scott.  I can tell you how to find this.

First of all, you want to go to the Dr. Math FAQ page,

http://mathforum.org/dr.math/faq/

and click on "Prime Numbers." Then click on "Mersenne Primes: History,
Theorems and Lists," and you will find a list of primes of the form
2^ - 1 giving the number of digits. You should see one with 386
digits.

Now if you multiply this by a number with 15 digits whose first two
digits are 10, you will get a number with 400 digits, because the
first digit of the big prime can easily be estimated to be no greater
than 6.

proceed to test the odd numbers one at a time until you find a prime.
To do the testing, go back to the "Prime Numbers" page and click on
"The Prime Pages"; then click on "Check a Number's Primality".

Good luck!

- Doctor Wilkinson, The Math Forum
http://mathforum.org/dr.math/
```

```
Date: 11/19/2005 at 22:58:06
From: Carmen
Subject: Prime Numbers Mega Multiplication

What is the product of (2^1279 - 1) * 100000000000031 = ??

I couldn't find the equipment to do the multiplication.

I found what I had to do from your prime page, as well as finding
the 15 digit prime, but I can't find a calculator that can accomodate
these huge numbers.
```

```
Date: 11/20/2005 at 09:51:12
From: Doctor Vogler
Subject: Re: Prime Numbers Mega Multiplication

Hi Carmen,

Thanks for writing to Dr Math.  I recommend the program GNU Pari,

http://pari.math.u-bordeaux.fr/

By the way, if you are trying to make a 400-digit product of two
primes, you can also pick any two numbers x and y whose product is
400 digits (such as two 200-digit numbers) and ask Pari for

nextprime(x)*nextprime(y).

For example, you could take

x = random(10^200)
y = random(10^200)
nextprime(x)*nextprime(y)

back and show me what you have been able to do, and I will try to
offer further suggestions.

- Doctor Vogler, The Math Forum
http://mathforum.org/dr.math/

```
Associated Topics:
High School Number Theory
Middle School Prime Numbers

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