Challenge Accepted

Find the smallest positive integer n such that
(i) n n has exactly 144 distinct positive divisors, and
(ii) there are ten consecutive integers among the positive divisors of n n .

(IMO)


The answer is 110880.

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2 solutions

Angshuman Podder
Sep 26, 2014

Among ten consecutive integers that divide n, there must exist numbers divisible by 2 3 , 3 2 , 5 , 7 , 11 2^3, 3^2, 5, 7,11 (since we are to find the smallest 10 consecutive divisors :P).

Thus the desired number has the form n = 2 α 1 × 3 α 2 × 5 α 3 × 7 α 4 × 1 1 α 5 2^{α_1}\times 3^{α_2} \times 5^{α_3} \times 7^{α_4} \times 11^{α_5} , where α 1 3 ; α 2 2 ; α 3 α 4 a 5 1 α_1 ≥ 3; α_2 ≥ 2; α_3 ≥ α_4\geq a_5\geq 1 .

Since n has ( α 1 + 1 ) ( α 2 + 1 ) ( α 3 + 1 ) . . . (α1 + 1)(α2 + 1)(α3 + 1)... distinct factors, and ( α 1 + 1 ) ( α 2 + 1 ) ( α 3 + 1 ) ( α 4 + 1 ) 48 (α_1 + 1)(α_2 + 1)(α_3 + 1)(α_4 + 1) ≥ 48 , we must have ( α 5 + 1 ) 3 (α_5 + 1) · · · ≤ 3 . Hence at most one a j > 4 a_{j>4} , Is positive, and in the minimal n this must be a 5 a_5 .

Checking through logic and some bashing, the possible combinations satisfying ( α 1 + 1 ) ( α 2 + 1 ) . . . ( α 5 + 1 ) = 144 (α_1 + 1)(α_2 + 1)...(α_5 + 1) = 144 one finds that the minimal n is 2 5 3 2 5 7 11 = 110880 2^5 · 3^2 · 5 · 7 · 11 = 110880 .

Moderator note:

This solution makes the assumption that 11 must divide the number.

See Dan Lawson's comment below.

I have edited both the latex and the wording a little in your solution. Please check that I did not alter the meaning of your solution in any way. You may edit your solution by clicking the tiny pencil in the top right of your solution. In doing so, you may also see the proper latex format for writing solutions and problems.

Overall, great problem. One of my favorites for sure. But is this really an IMO question? It seems a little too easy.

Trevor Arashiro - 6 years, 8 months ago

Having 10 consecutive divisors doesn't imply that 11 is a divisor. The extra prime divisor comes as a result of the minimization requirement.

Dan Lawson - 6 years, 8 months ago

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Can you add a solution to explain how you arrived at "extra prime divisor comes as a result of the minimization requirement"?

Calvin Lin Staff - 6 years, 8 months ago

yup .IMO for sure though I don't recall the year..and thanx for the edit ... I will be posting some good problems soon which i encountered in my 10+2 level...

Angshuman Podder - 6 years, 8 months ago

1 is divisor or not... because in 10 consecutive no need of 11

Vijay Singh Nishad - 6 years, 8 months ago

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.... but if you remove 11, you need to allocate another number, which has to be >11 , if you do so, you will get a greater no. than 110880!!

jaiveer shekhawat - 6 years, 8 months ago

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1,2,3,4,5,6,7,8,9,10 10 Consecutive 2^a 3^b 5^c 7^d where a>3 and b>2 now solve

Vijay Singh Nishad - 6 years, 8 months ago

Why must you allocate another number? How do you know that?

Calvin Lin Staff - 6 years, 8 months ago

nice problem just couldn't crack 32 and got 11^2 in place of it

akash deep - 6 years, 8 months ago
Jaya Yarlagadda
Oct 1, 2014

S i m p l e S o l u t i o n F o r : X F r o m ( i i ) : S i n c e t h e r e a r e 10 c o n s e c u t i v e i n t e g e r s i n d i v i s o r s , t h e n u m b e r s h o u l d b e d i v i s i b l e b y 2 , 3 , 5 , 7 ( p r i m e s w h o s e m u l t i p l e s p r e s e n t i n a n y 10 c o n s e c u t i v e n o . ) s o X s h o u l d b e s o m e t h i n g l i k e 2 p × 3 q × 5 r × 7 s l e a s t n o o f t h i s k i n d i s 2520 ( h a s 10 c o n s e c u t i v e n o s a s d i v i s o r s 1 10 ) 2520 = 2 3 × 3 2 × 5 × 7 C o u n t ( D i v i s o r s ) = ( 3 + 1 ) × ( 2 + 1 ) × ( 1 + 1 ) × ( 1 + 1 ) = 4 × 3 × 2 × 2 144 ( I ) F r o m ( i ) : N o o f d i v i s o r s = 144 = 3 × 3 × 4 × 4 W e c a n m u l t i p l y 2520 w i t h l e a s t n u m b e r & a l s o C o u n t ( D i v i s o r s ) = 144 T w o w a y s t o d o t h a t 1. U s i n g s a m e p r i m e s 2 3 × 3 3 × 5 2 × 7 2 = 262400 ( w i t h 144 d i v i s o r s ) [ L e a s t p o s s i b l e n o . ] M u l t i p l i e d 2520 w i t h 35 2. U s e o n e m o r e p r i m e 2 5 × 3 2 × 5 × 7 × 11 = 110880 ( w i t h 144 d i v i s o r s ) [ L e a s t p o s s i b l e n o . ] M u l t i p l i e d w i t h 44 L e a s t P o s s i b l e X = 110880 ( w i t h 144 d i v i s o r s ) Simple\quad Solution\quad For:\quad X\\ From\quad (ii):\\ Since\quad there\quad are\quad 10\quad consecutive\quad integers\quad in\quad divisors,\quad the\quad number\\ should\quad be\quad divisible\quad by\quad 2,3,5,7\quad (primes\quad whose\quad multiples\quad present\quad in\quad any\quad 10\quad consecutive\quad no.)\quad \\ so\quad X\quad should\quad be\quad something\quad like\quad { 2 }^{ p }\times { 3 }^{ q }\times { 5 }^{ r }\times { 7 }^{ s }\quad -\quad least\quad no\quad of\quad this\quad kind\quad is\quad 2520\quad (has\quad 10\quad consecutive\quad no's\quad as\quad divisors\quad 1-10)\\ 2520\quad =\quad { 2 }^{ 3 }\times { 3 }^{ 2 }\times { 5 }\times { 7 }\quad \quad \Rightarrow \quad Count(Divisors)\quad =\quad (3+1)\times (2+1)\times (1+1)\times (1+1)\quad =\quad 4\times 3\times 2\times 2\quad \neq \quad 144\quad \quad --\quad (I)\\ From\quad (i):\quad \\ No\quad of\quad divisors\quad =\quad 144\quad =\quad 3\times 3\times 4\times 4\\ We\quad can\quad multiply\quad 2520\quad with\quad least\quad number\quad \& \quad also\quad Count(Divisors)=144\\ Two\quad ways\quad to\quad do\quad that\quad \\ 1.\quad Using\quad same\quad primes\quad { 2 }^{ 3 }\times { 3 }^{ 3 }\times { { 5 }^{ 2 } }\times { { 7 }^{ 2 } }\quad =\quad 262400\quad (with\quad 144\quad divisors)\quad [Least\quad possible\quad no.]\quad \vdots \quad Multiplied\quad 2520\quad with\quad 35\\ 2.\quad Use\quad one\quad more\quad prime\quad { 2 }^{ 5 }\times { 3 }^{ 2 }\times { 5 }\times { 7 }\times 11\quad =\quad 110880\quad (with\quad 144\quad divisors)\quad [Least\quad possible\quad no.]\quad \vdots \quad Multiplied\quad with\quad 44\\ Least\quad Possible\quad X\quad =\quad 110880\quad (with\quad 144\quad divisors)

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