# 792 Number Facts and Factors of the Year 2017

Happy New Year, everybody!

So much of what I’ve included in this post is a bit difficult for elementary school students, but here is an area problem that they should be able to do:

Perhaps you’ll recognize that as just another way to illustrate that 44² + 9² = 2017.

This area problem based on 33² + 28² + 12² = 2017 will be a little bit more challenging for students:

2016 had more factors, positive and negative, than anybody could have imagined, but 2017 is a prime number year, so hopefully it will be filled with less drama.

I have a lot to say about the number 2017 with a little help from twitter.

2017 has only TWO positive real factors, 1 and 2017, and only TWO negative real factors, -1 and -2017. Positive or negative, ALL the real possible factor pairs for 2017 are

• 1 x 2017 and (-1) x (-2017).

2017 = 4(504) + 1. So 2017 is the sum of two squares. Which ones?

44² + 9² = 2017. That sum-of-squares number fact means that 2017 is the hypotenuse of a Pythagorean triple, specifically, 792-1855-2017. Since 2017 is a prime number, this triple is also a primitive.

Here’s how those numbers were calculated from the fact that  44² + 9² = 2017:

• 2(44)(9) = 792
• 44² – 9² = 1855
• 44² + 9² = 2017

2017 is also the short leg in a rather monstrous primitive Pythagorean triple:

• 2017² + 2,034,144² = 2,034,145²

Hmm. 44² + 9² = 2017 means we can find some COMPLEX factor pairs for 2017:

• (44 + 9i)(44 – 9i) = 2017
• (-44 + 9i)(-44 – 9i) = 2017
• (9 + 44i)(9 – 44i) = 2017
• (-9 + 44i)(-9 – 44i) = 2017

Who knows what all those COMPLEX factors will bring to the coming year? Each of them was derived from the fact that 44² + 9² = 2017.

2017 is the sum of THREE squares three different ways:

• 37² + 18² + 18²  = 2017
• 33² + 28² + 12² = 2017
• 30² + 26² + 21² = 2017

Since 37² = 35² + 12², 30² = 24² + 18², and 26² = 24² + 10², we can write 2017 as the sum of these squares, too:

• 35² + 18² + 18² + 12² = 2017
• 30² + 24² + 21² + 10² = 2017
• 26² + 24² + 21² + 18² = 2017
• 24² + 24² + 21² + 18² + 10² = 2017

Here’s more sums and/or differences of squares from twitter:

This representation of 2017 is “two” powerful:

2017 is a PALINDROME in bases 31, 32, and 36:

• 232 BASE 31; note that 2(31²) + 3(31) + 2(1) = 2017
• 1V1 BASE 32 (V is 31 base 10); note that 1(32²) + 31(32) + 1(1) = 2017
• 1K1 BASE 36 (K is 20 base 10); note that 1(36²) + 20(36) + 1(1) = 2017

Also Stetson.edu tells us this 2017 fact about the totient function: “2017 is a value of n for which φ(n) = φ(n-1) + φ(n-2).”

PROOF that 2017 is a prime number:

We can use these three facts to help us verify that 2017 is a prime number:

1. 2017 = 4(504) + 1
2. 44² + 9² = 2017
3. √2017 ≈ 44.8998886

Since 2017 can be written as one more than a multiple of four, and 44 and 9 have no common prime factors, 2017 will be a prime number unless it is divisible by 5, 13, 17, 29, 37, or 41 (all the prime numbers less than √2017 ≈ 44.9 that have a remainder of one when divided by 4). That’s right, we ONLY have to divide 2017 by those SIX numbers to verify that it is prime: (Read here for why this is true.)

• 2017 ÷ 5 = 403.4
• 2017 ÷ 13 ≈ 155.15
• 2017 ÷ 17 ≈ 118.65
• 2017 ÷ 29 ≈ 69.55
• 2017 ÷ 37 ≈ 54.51
• 2017 ÷ 41 ≈ 49.20

We don’t get a whole number answer for any of those divisions, so 2017 is prime!

If you’re looking for more reasons to be interested in the number 2017, read David Radcliffe’s article:

Here are some other lovely mathematical thoughts about 2017 that I found on twitter:

In case you don’t know (and apparently a lot of people don’t know), 0! = 1, so this expression really does equal 2017.

And finally, if you click on this next link, David Mitchell will explain the tessellation of the number 2017.

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Since this is my 792nd post, I’ll write a little bit about the number 792:

792 has 24 factors so I’ll include a few of its many possible factor trees:

Since 792 has so many factors, it is a leg in MANY Pythagorean triples. Here I list some factors that generate PRIMITIVE triples:

• 2(396)(1): 792-156815-156817
• 2(99)(4): 792-9785-9817
• 2(44)(9): 792-1855-2017 (illustrated near the top of this post)
• 2(36)(11): 792-1175-1417

792 is the sum of three squares four different ways including three ways that repeat squares:

• 28² + 2² + 2² = 792
• 26² + 10² + 4² = 792
• 20² + 14² + 14² = 792
• 18² + 18² + 12² = 792

792 is a palindrome in bases 32 and 35:

• OO BASE 32 (O is 24 base 10); note that 24(32) + 24(1) = 792
• MM BASE 35 (M is 22 base 10); note that 22(35) + 22(1) = 792

Finally here is the factoring information for the number 792:

• 792 is a composite number.
• Prime factorization: 792 = 2 x 2 x 2 x 3 x 3 x 11, which can be written 792 = (2^3) x (3^2) x 11
• The exponents in the prime factorization are 3, 2 and 1. Adding one to each and multiplying we get (3 + 1)(2 + 1)(1 + 1) = 4 x 3 x 2 = 24. Therefore 792 has exactly 24 factors.
• Factors of 792: 1, 2, 3, 4, 6, 8, 9, 11, 12, 18, 22, 24, 33, 36, 44, 66, 72, 88, 99, 132, 198, 264, 396, 792
• Factor pairs: 792 = 1 x 792, 2 x 396, 3 x 264, 4 x 198, 6 x 132, 8 x 99, 9 x 88, 11 x 72, 12 x 66, 18 x 44, 22 x 36 or 24 x 33

Taking the factor pair with the largest square number factor, we get √792 = (√36)(√22) = 6√22 ≈ 28.14249.

# 791 and Level 4

To see if 791 is divisible by 7, you could try either one of these divisibility tricks:

• 791 is divisible by 7 because 79 – 2(1) = 77 which obviously is divisible by 7.
• 791 is divisible by 7 because 79 + 5(1) = 84 which most people know is 12 × 7.

Print the puzzles or type the solution on this excel file: 10-factors-788-794

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• 791 is a composite number.
• Prime factorization: 791 = 7 x 113
• The exponents in the prime factorization are 1 and 1. Adding one to each and multiplying we get (1 + 1)(1 + 1) = 2 x 2 = 4. Therefore 791 has exactly 4 factors.
• Factors of 791: 1, 7, 113, 791
• Factor pairs: 791 = 1 x 791 or 7 x 113
• 791 has no square factors that allow its square root to be simplified. √791 ≈ 28.12472222.

791 is the hypotenuse of Pythagorean triple 105-784-791 which is 15-112-113 times 7.

791 is also the sum of seven consecutive prime numbers:

• 101 + 103 + 107 + 109 + 113 + 127 + 131 = 791

# 790 and Level 3

• 790 is a composite number.
• Prime factorization: 790 = 2 x 5 x 79
• The exponents in the prime factorization are 1, 1, and 1. Adding one to each and multiplying we get (1 + 1)(1 + 1)(1 + 1) = 2 x 2 x 2 = 8. Therefore 790 has exactly 8 factors.
• Factors of 790: 1, 2, 5, 10, 79, 158, 395, 790
• Factor pairs: 790 = 1 x 790, 2 x 395, 5 x 158, or 10 x 79
• 790 has no square factors that allow its square root to be simplified. √790 ≈ 28.106939.

Here is today’s puzzle:

Print the puzzles or type the solution on this excel file: 10-factors-788-794

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Here’s a little more about the number 790:

There are four ways to make 790 using three squares:

• 21² + 18² + 5² = 790
• 27² + 6² + 5² = 790
• 23² + 15² + 6² = 790
• 22² + 15² + 9² = 790

790 is the hypotenuse of Pythagorean triple 474-632-790 which is 3-4-5 times 158.

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# 789 and Level 2

789 consists of exactly three consecutive numbers so it is divisible by 3.

Print the puzzles or type the solution on this excel file: 10-factors-788-794

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• 789 is a composite number.
• Prime factorization: 789 = 3 x 263
• The exponents in the prime factorization are 1 and 1. Adding one to each and multiplying we get (1 + 1)(1 + 1) = 2 x 2 = 4. Therefore 789 has exactly 4 factors.
• Factors of 789: 1, 3, 263, 789
• Factor pairs: 789 = 1 x 789 or 3 x 263
• 789 has no square factors that allow its square root to be simplified. √789 ≈ 28.08914.

789 is the sum of consecutive prime numbers 2 different ways:

• 257 + 263 + 269 = 789 (that’s 3 consecutive primes.)
• 3 + 5 + 7 + 11 + 13 + 17 + 19 + 23 + 29 + 31 + 37 + 41 + 43 + 47 + 53 + 59 + 61 + 67 + 71 + 73 + 79 = 789 (that’s 21 consecutive primes!)

789 is the sum of three squares eight different ways:

• 28² + 2² + 1² = 789
• 26² + 8² + 7² = 789
• 25² + 10² + 8² = 789
• 23² + 16² + 2² = 789
• 23² + 14² + 8² = 789
• 22² + 17² + 4² = 789
• 22² + 16² + 7² = 789
• 20² + 17² + 10² = 789

# 788 and Level 1

Since 88, its last two digits, are divisible by 4, we know that 788 and every other whole number ending in 88 is divisible by 4.

I learned the following fascinating fact about these six numbers starting with 788 from Stetson.edu:

788 is also palindrome 404 in BASE 14. Note that 4(196) + 0(14) + 4(1) = 788.

788 is the hypotenuse of Pythagorean triple 112-780-788 which is 28-195-197 times 4.

Print the puzzles or type the solution on this excel file: 10-factors-788-794

• 788 is a composite number.
• Prime factorization: 788 = 2 x 2 x 197, which can be written 788 = (2^2) x 197
• The exponents in the prime factorization are 2 and 1. Adding one to each and multiplying we get (2 + 1)(1 + 1) = 3 x 2  = 6. Therefore 788 has exactly 6 factors.
• Factors of 788: 1, 2, 4, 197, 394, 788
• Factor pairs: 788 = 1 x 788, 2 x 394, or 4 x 197
• Taking the factor pair with the largest square number factor, we get √788 = (√4)(√197) = 2√197 ≈ 28.071338.

# 787 Always a Unique Solution

• 787 is a prime number.
• Prime factorization: 787 is prime.
• The exponent of prime number 787 is 1. Adding 1 to that exponent we get (1 + 1) = 2. Therefore 787 has exactly 2 factors.
• Factors of 787: 1, 787
• Factor pairs: 787 = 1 x 787
• 787 has no square factors that allow its square root to be simplified. √787 ≈ 28.05352.

Now for today’s puzzle….The fact that these Find the Factor puzzles always have a unique solution is an important clue in solving this rather difficult puzzle. Good luck!

Print the puzzles or type the solution on this excel file: 12-factors-782-787

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Here’s more about the number 787:

787 is a palindrome in bases 4, 10, 11 and 16:

• 30103 BASE 4; note that 3(256) + 0(64) + 1(16) + 0(4) + 3(1) = 787
• 787 BASE 10; note that 7(100) + 8(10) + 7(1) = 787
• 656 BASE 11; note that 6(121) + 5(11) + 6(1) = 787
• 313 BASE 16; note that 3(256) + 1(16) + 3(1) = 787

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What did I mean when I wrote that the puzzles always having a unique solution is an important clue? There is only one clue in the puzzle that is divisible by 11. One of the rows and one of the columns do not have a clue, so the other 11 will go with one of them. The cell where the empty row and empty column intersect cannot be 132 because if that worked, it would produce two possible solutions to the puzzle. This table explains a logical order to find the solution.

# 786 and Level 5

786 is even so it is divisible by 2. Also since 786 is made from 3 consecutive numbers, we can tell automatically that it is divisible by 3. Those two facts together mean 786 is also divisible by 6.

• 786 is a composite number.
• Prime factorization: 786 = 2 x 3 x 131
• The exponents in the prime factorization are 1, 1, and 1. Adding one to each and multiplying we get (1 + 1)(1 + 1)(1 + 1) = 2 x 2 x 2 = 8. Therefore 786 has exactly 8 factors.
• Factors of 786: 1, 2, 3, 6, 131, 262, 393, 786
• Factor pairs: 786 = 1 x 786, 2 x 393, 3 x 262, or 6 x 131
• 786 has no square factors that allow its square root to be simplified. √786 ≈ 28.03569.

Today’s Find the Factors puzzle:

Print the puzzles or type the solution on this excel file: 12-factors-782-787

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Here’s a little more about the number 786:

786 is 123 in BASE 27 because 1(27²) + 2(27) + 3(1) = 786.

786 is the sum of two consecutive primes: 389 + 397 = 786

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# 785 and Level 4

• 785 is a composite number.
• Prime factorization: 785 = 5 x 157
• The exponents in the prime factorization are 1 and 1. Adding one to each and multiplying we get (1 + 1)(1 + 1) = 2 x 2 = 4. Therefore 785 has exactly 4 factors.
• Factors of 785: 1, 5, 157, 785
• Factor pairs: 785 = 1 x 785 or 5 x 157
• 785 has no square factors that allow its square root to be simplified. √785 ≈ 28.01785.

Now for today’s puzzle:

Print the puzzles or type the solution on this excel file: 12-factors-782-787

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Here’s a little more about the number 785:

785 is the sum of two squares two different ways:

• 28² + 1² = 785
• 23² + 16² = 785

785 is also the sum of three squares three different ways:

• 26² + 10² + 3² = 785
• 25² + 12² + 4² = 785
• 19² + 18² + 10² = 785

Because its prime factorization is 5 × 157 (two numbers that are also the sum of two squares), 785 is the hypotenuse of four Pythagorean triples, two of which are primitive triples:

• 56-783-785 primitive calculated from 2(28)(1), 28² – 1², 28² + 1²
• 273-736-785 primitive calculated from 23² – 16², 2(23)(16), 23² + 16²
• 425-660-785 which is 5 times 85-132-157
• 471-628-785 which is 157 times 3-4-5

785 is also a palindrome in two different bases:

• 555 BASE 12; note that 5(144) + 5(12) + 5(1) = 785
• 101 BASE 28; note that 1(28²) + 0(28) + 1(1) = 785

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# 784 and Level 3

If you know the multiplication facts up to 12 x 12, then it is obvious that 784 is divisible by 7. However, 784 is divisible by a whole lot more numbers than that. 784 can actually be evenly divided by 15 different numbers. Hmm, 15, that’s an odd number. A number’s factors always come in pairs. One of its factor pairs must contain the same factor twice, meaning 784 is a perfect square!

• 784 is a composite number.
• Prime factorization: 784 = 2 x 2 x 2 x 2 x 7 x 7, which can be written 784 = (2^4) x (7^2)
• The exponents in the prime factorization are 4 and 2. Adding one to each and multiplying we get (4 + 1)(2 + 1) = 5 x 3 = 15. Therefore 784 has exactly 15 factors.
• Factors of 784: 1, 2, 4, 7, 8, 14, 16, 28, 49, 56, 98, 112, 196, 392, 784
• Factor pairs: 784 = 1 x 784, 2 x 392, 4 x 196, 7 x 112, 8 x 98, 14 x 56, 16 x 49, or 28 x 28
• 784 is a perfect square. √784 = 28

But that’s not the only thing remarkable about this perfect square: √784 is 28, the 7th triangular number, so like all other squared triangular numbers 784 has this additional property:

Just as 784 is a perfect square, five of the twelve clues in today’s puzzle are also perfect squares. But don’t let that fact trick you into writing the same factor in both the first column and the top row every time!

Print the puzzles or type the solution on this excel file: 12-factors-782-787

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Here’s more about the number 784:

784 can be written as the sum of three squares in exactly one way:

• 24² + 12² + 8² = 784

784 is also a palindrome in Bases 13, 17, and 27:

• 484 BASE 13; note that 4(169) + 8(13) + 4(1) = 784
• 2C2 BASE 17 (C is 12 base 10); note that 2(289) + 12(17) + 2(1) = 784
• 121 BASE 27; note that 1(27²) + 2(27) + 1(1) = 784

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# 783 and Level 2

• 783 is a composite number.
• Prime factorization: 783 = 3 x 3 x 3 x 29, which can be written 783 = (3^3) x 29
• The exponents in the prime factorization are 3 and 1. Adding one to each and multiplying we get (3 + 1)(1 + 1) = 4 x 2 = 8. Therefore 783 has exactly 8 factors.
• Factors of 783: 1, 3, 9, 27, 29, 87, 261, 783
• Factor pairs: 783 = 1 x 783, 3 x 261, 9 x 87, or 27 x 29
• Taking the factor pair with the largest square number factor, we get √783 = (√9)(√87) = 3√87 ≈ 27.982137.

Here’s today’s puzzle. It’s a level 2 so it isn’t very difficult:

Print the puzzles or type the solution on this excel file: 12-factors-782-787

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27 x 29 = 783. Since (n – 1)(n + 1) always equals n² – 1, we know that 783 is one number away from the next perfect square.

29 is a factor of 783, making 783 the hypotenuse of a Pythagorean triple:

• 540-567-783, which is 27 times 20-21-29.
• Thus 540² + 567² = 783² just as 20² + 21² = 29².

783 is also a palindrome in bases 15, 23, and 28:

• 373 BASE 15; note that 3(225) + 7(15) + 3(1) = 783
• 1B1 BASE 23 (B is 11 base 10); note that 1(23²) + 11(23) + 1(1) = 783
• RR BASE 28 (R is 27 base 10); note that 27(28) + 27 = 783

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