Such Big, Much Scare, Many Easy!

Given that 1002004008016032 1002004008016032 has a prime factor " P P " which is greater than 250000 250000 , find P P

Note-Computational devices are not allowed. This problem is taken from an old math contest. If you're the one who's gonna use a program/ a super-calculator to churn out the answer -you're most welcome to stay away!

If you solve this problem, Please post solutions too!


The answer is 250501.

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4 solutions

Karthik Sharma
Jul 21, 2014

By multiplying the 1st 7 digits by 8 we get the last 9 digits as result.

\Rightarrow 1002004 1002004 is a factor of the 1002004008016032 1002004008016032

And, 1002004 = 2 × 2 × 250501 1002004 = 2\times 2\times 250501

Given that P > 250000 P>250000 , And P is Prime,

250501 \boxed{250501} is the answer.

Great intuition!!!!!

Krishna Ar - 6 years, 10 months ago

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Thanks!

The zeroes between the digits made it easy

Karthik Sharma - 6 years, 10 months ago

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Hi! Are you in 11th std or 10th?

Krishna Ar - 6 years, 10 months ago

but how your brain goes in that direction???please tell i want also to be a genius like you @Karthik Sharma

sakshi rathore - 5 years, 11 months ago

wow very nice

i use calc x_x

math man - 6 years, 8 months ago
Joseph Varghese
Aug 17, 2014

S p l i t i t u p a n d w r i t e n e a t l y t o g e t : Split\quad it\quad up\quad and\quad write\quad neatly\quad to\quad get:

1002004008016032 = 2 0 10 15 + 2 1 10 12 + 2 2 10 9 + 2 3 1 0 6 + 2 4 10 3 + 2 5 1002004008016032\ =\ { 2 }^{ 0 }*{ 10 }^{ 15 }+{ 2 }^{ 1 }*{ 10 }^{ 12 }+{ 2 }^{ 2 }*{ 10 }^{ 9 }+{ 2 }^{ 3 }*10^{ 6 }+{ 2 }^{ 4 }*{ 10 }^{ 3 }+{ 2 }^{ 5 }

A l s o n o t i c e t h a t Also\quad notice\quad that

( x 6 y 6 ) ( x y ) = ( x + y ) ( x 2 + x y + y 2 ) ( x 2 x y + y 2 ) \frac { { (x }^{ 6 }-{ y }^{ 6 }) }{ (x-y) } =(x+y)({ x }^{ 2 }+xy+{ y }^{ 2 })({ x }^{ 2 }-xy+{ y }^{ 2 })

= x 5 + y x 4 + y 2 x 3 + y 3 x 2 + y 4 x + y 5 ={ x }^{ 5 }+{ yx }^{ 4 }+{ { y }^{ 2 }x }^{ 3 }+{ y }^{ 3 }{ x }^{ 2 }+{ y }^{ 4 }x+{ y }^{ 5 }

N o w l e t x = 10 3 a n d y = 2 S o , Now\quad let\quad x={ 10 }^{ 3 }\quad and\quad y=2\\ So,

1002004008016032 = 1002 998004 1002004 = 1002 4 249501 4 250501 1002004008016032=1002*998004*1002004=1002*4*249501*4*250501

A s b y t h e q u e s t i o n t h e r e i s a p r i m e P > 250000. As\quad by\quad the\quad question\quad there\quad is\quad a\quad prime\quad P>250000.

S o w i t h o u t f u r t h e r p r o v i n g t h a t 250501 i s a p r i m e , So\quad without\quad further\quad proving\quad that\quad 250501\quad is\quad a\quad prime,

w e c a n a s s u m e t h a t P = 250501 we\quad can\quad assume\quad that\quad \boxed { P=250501 }

It is an extremely easy problem. Took only a few seconds to solve. It should have been a level 2 problem.

Jared Low
Jan 2, 2015

Combining elements of both Karthik Sharma 's and Joseph Varghese 's solutions:

We first note that 3 , 7 3, 7 divides our number, which will be important later.

We now consider making use of factorisation below:

100 0 5 + 100 0 4 2 1 + + 100 0 1 2 4 + 2 5 = 100 0 6 2 6 1000 2 1000^5+ 1000^4*2^1+\cdots +1000^1*2^4+2^5=\frac{1000^6-2^6}{1000-2}

= ( 1000 + 2 ) ( 100 0 2 + 1000 2 + 2 2 ) ( 100 0 2 1000 2 + 2 2 ) =(1000+2)(1000^2+1000*2+2^2)(1000^2-1000*2+2^2)

= 1002 998004 1002004 =1002*998004*1002004

And we note that 1002 = 2 3 167 1002=2*3*167 , 998004 = 4 3 7 11881 998004=4*3*7*11881 , since as we've observed since 3 , 7 3,7 do not divide 1002004 1002004 , they must divide some of the other factors.

1002004 = 4 250501 1002004=4*250501 , which so far is the only non-factored factor of out number that we have so far that is > 250000 >250000

(Side note: 11881 11881 further factorises to 10 9 2 109^2 , but that's unimportant as it is already < 250000 <250000 )

Hence, our desired factor is thus 250501 \boxed{250501}

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