2 2 0 1 4 + 1 0 0 7 4 Is this sum equal to a prime number?
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what is this whole mod thing? Would FP1 mean I understand it or would I need higher? Fp2/3/ undergrad? this mod thing comes up lots and i dont get it.
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Modulus: The remainder after division. 17 MOD 5 = 2 because 17/5 = 3 with a remainder of 2.
It means modulus. And I would say you should know it by now. I'm probably younger than you.
i did the same
Well. The way I figured it out was similar. Looking at only the units digit, 7×7×7×7 units digit goes from 7 to 9 to 3 to 1. As for 2^2014, it goes from 2 to 4 to 8 to 6. So 2014 divided by 4 gives a remainder of 2, thus 4+1=5 it will be a larger number with units digit as 5 and so it will not be prime
You might want to change your latex.
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LOL, didn't see that.
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I should blame myself for not noticing this easy method would work instead of the Sophie Germain identity.
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@Joshua Ong – Well, essentially Sharky's solution is proving a specific case in the general solution of proving n 4 + 4 n is not prime.
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@Daniel Liu – Yes, exactly why I said Joshua's solution is better
@Daniel Liu – is it true that n^4 +4^n is not a prime for all n? Just clarify my doubt
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@Girish Chukka – Only for positive integers n greater than 1.
yes, I did with the same method.
The unit integers of this summation is 7, how can it be divisible by 5. Pls correct me if I have missed any point.
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Last digit of 2^2014 is 4 and that of 1007^4 is 1 ,therefore last digit of sum is 5
2 2 0 1 4 + 1 0 0 7 4 = 1 0 0 7 4 + 4 ( 2 5 0 3 ) 4
Now we can factorise this using the Sophie-Germain Identity:
( 1 0 0 7 2 + 2 ( 2 5 0 3 ) 2 + 2 ( 1 0 0 7 ) ( 2 5 0 3 ) ) ( 1 0 0 7 2 + 2 ( 2 5 0 3 ) 2 − 2 ( 1 0 0 7 ) ( 2 5 0 3 ) )
Thus it is clear that the given number is not a prime.
Not necessarily; you still have to prove both factors are integers greater than 1. (Otherwise you can have something like 5 = 4 ⋅ 1 4 + 1 4 = ( 1 2 + 2 ⋅ 1 ⋅ 1 + 2 ⋅ 1 2 ) ( 1 2 − 2 ⋅ 1 ⋅ 1 + 2 ⋅ 1 2 ) and thus claim 5 is not prime.)
True, but that proof is obvious: 2^503 > 1007
This is a very nice way ....I liked it
you still have to prove both factors are integers greater than 1.
Wow nice proof! Didn't know about the Sophie-Germain identity so I used modulus.
Would you please tell me detail about \color\red\Sophie- Germain Identity ?
2 2 0 1 4 ≅ 4 m o d 5 and 1 0 0 7 4 ≅ 1 m o d 5 thus 5 ∣ 2 2 0 1 4 + 1 0 0 7 4
This is an elegant way to prove its not a prime but we can also see that 5 is a divisor of this number
to say that the given number is a prime or composite, each of the numbers in both the braces must be confirmed as primes or as composites but not be left as some big random numbers.
It's a prime number. Let 2^2014+1007^4 = 4^1007 + 1007^4 =x^y +y^x (x is even & y is odd) Let, 2^3 + 3^2 =17 ---prime 2^5 + 5^2 = 57---prime 2^7 +7^2 = 177---prime 4^5 + 5^4 = 4405---- prime It means Even^odd + Odd^even = prime Hence, from above 4^1007 + 1007^4 ---- is prime
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Karthik's solution using the Sophie-Germain Identity shows it is composite. Your supposition that e v e n o d d + o d d e v e n = p r i m e is false. Your own example of 4 5 + 5 4 = 1 6 4 9 disproves your supposition because 1 6 4 9 = 1 7 ⋅ 9 7 .
How are 57 (3 x 19), 177 (3 x 59) and 4405 (5 x 881) prime numbers ?
Note that 2 2 0 1 4 + 1 0 0 7 4 = 4 1 0 0 7 + 1 0 0 7 4 . The rest is left as an exercise to the reader.
Yup n^4 + 4^n can never be a prime. An elementary application of sg identity :). Btw, most of these type questions have answers in the negative :P
EDIT- Exception for n=1
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Why can it never be prime?
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Sophie Germain (SG) Identity: a^4+4b^4 = a^4 + 4b^4 = (a^2 + 2b^2 + 2ab)(a^2 + 2b^2 - 2ab)
n^4+4^n is never prime. Proof: If n is even, then the sum is even and we are done. Since odd integers are in form 2k+1, we may perform the transformation: n^4+4^n = n^4 + 4^2k+1 = n^4 + 4(4^2k) = n^4 + 4(2^k)^4 Now, by Sophie-Germain, we are also done.
Can you elaborate on the identity for our learning? Thanks.
What is sg identity?
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sophie germain. when you have a^4+4b^4, you factor as (a^2+2b^2+2ab)(a^2+b^2-2ab)
last digit of 2^2014 is 4 and that of 1007^4 is 1. So, the last digit of the sum is 4+1=5 and a number ending with 5 can never be prime (except 5 itself).
It's very simple..both 2 and 7 repeat their last digits for every 4 powers.
So, if we divide the power of 2 by 4, the remainder is 2. So the last digit of 2^2014 = 4 And, if we divide the power of 7 by 4, the remainder is 0. So the last digit of 7^1004 = 1
So on adding 2^2014 and 7^1004, we get the last digit = 4+1 = 5 And, since every number ending with 5 is divisible by 5, Therefore 2^2014 + 7^1004 is NOT PRIME!!!
From Fermat's Little Theorem, 2 2 0 1 4 ≡ 2 2 = 4 (mod 5) Also, 1 0 0 7 4 ≡ 2 4 = 1 6 ≡ 1 (mod 5) Therefore, 2 2 0 1 4 + 1 0 0 7 4 ≡ 0 (mod5) Which proves its divisibility by 5, which makes it not a prime.
Since
2^2014+1007^4
= (2^1007)^2+(1007^2)^2
= (2^1007+1007^2)^2-2(2^1007)(1007^2)
= (2^1007+1007^2)^2-[1007(2^504)]^2
= [2^1007+1007^2-1007(2^504)] * [2^1007+1007^2+1007(2^504)]
= [1007^2+(2^504)(2^503-1007)] * [2^1007+1007^2+1007(2^504)]
is a product of two positive integers, so it is composite.
we can do it this way also....
last digit of 2^2014 = 4
last digit of 1007^4 = 1
addition of these two numbers would bear unit digit as 5.
hence it is definitely not a prime no.
This is the cleanest, most concise and eligant proof offered on this problem. Nice job!
Its 4^X +X^4 where X is 1007. All odd powers of 4 end in a 4. 1007 to the fourth ends in a 1. So the sum must end in a 5 and be divisible by 5.
Good solution
1007^4 = (1000 + 7)^4- the final number is 1 as 7^4 = 2401 2^2014 = 4^1007. and alternatly powers come with end digits 4 and 6.. so if 4^(2n), the digit is 6 and 4^(2n+1) digit is 4, so last digit 4+1=5 hence not prime but a composite no.
2 no matter power wat also even 7 power wat always odd Even + odd =odd
This problem can be approached logically. We know 2^2014 is not prime since whatever that number is would be divisible by 2. Also, 1007^4 is not prime since the resulting number is divisible by 1007. As a result, we have the sum of 2 composite numbers which is always composite.
The sum of two composites is not always a composite. Note 4+9=13
2^2014 + 1007^4
= 4^1007 + 1007^4
= (4^1007 + 1) + (1007^4 -1)
Then, by lifting the exponents up:
(4^1007 + 1) + (1007^4 -1)
= (4 + 1)(4^1006 - 4^1005 + ... + 1) + (1007^2 + 1)(1007 + 1)(1007 - 1)
5 is a factor of (1007^2 + 1), because the last two digits of 1007^2 are 49.
Therefore, 5 is a factor of both terms in the expression.
2 2 0 1 4 ≡ 4 1 0 0 7 ≡ ( − 1 ) 1 0 0 7 ≡ − 1 ( m o d 5 ) and
1 0 0 7 4 ≡ 2 4 ≡ 1 ( m o d 5 )
So their sum must be a multiple of 5, hence not a prime.
2014 = 4 Mod 10 1007 = 7 Mod 10
2^4 + 7^4 = 6 + 9 Mod 10
15 Mod 10 = 5 !!!!
2^2014 has the last digit of 4. 1007^4 has the last digit of 1. Both of them will be summed up to a number ending with 5. This is not possible to be a prime.
aha.. this is just the sophie germains identity.. just write 2^2014 as 4^1007
My Proof: in that case, the general eq. is 2^(2x) + x^4
try x = 3 you'll get, 145
try x = 2 you'll get 32
try x = 4 you'll get, 512
So, it means, if x is a positive integer and it is not = to 1, always, it's a COMPOSITE #. therefore, the given eq. is COMPOSITE so, NO is the ANSWER.. :D
We can use the Last Digit Pattern to solve this problem.
The last digit of 2 2 0 1 4 is 4,
while the last digit of 1 0 0 7 4 is 1.
1+4 = 5, so the number is not a prime.
simple 2^4n+2(n=503 here)will have last digit 4 and 7^4 has last digit 1 1+4 = 5. if the last digit is 5 then it is divisible by 5
unit digit wil be 5.so it will be definitely divisible by 5
This number is divisible by 5 . If we divide it by 5 and see the remainder of both the power terms , they add up to 5 .......therefore its not a prime
when two non prime no. are added their solution is non prime...so the answer is non prime...:)
What about 10 + 9?
Not always...4+9 is a prime
Not always, as Veera and Alija demonstrate; however, the sum of two positive integers that have a common prime divisor, is composite.
As we can find the last digit of the term: (4+1) = 5 ( only the last digit....)
which is clearly divisible by 5
Rabin-Miller primality test with BigIntegers in Java: http://ideone.com/zaiCP6
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@Joshua Ong 's solution is way better but here is something I noticed.
2 2 0 1 4 = 4 ( m o d 1 0 ) and 1 0 0 7 4 = 1 ( m o d 1 0 )
4 + 1 = 5
Clearly, this number must be divisible by 5. Like I said, Joshua's is better.