(i)
n
has exactly 144 distinct positive divisors, and
(ii) there are ten consecutive integers among the positive divisors of
n
.
(IMO)
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This solution makes the assumption that 11 must divide the number.
See Dan Lawson's comment below.
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Overall, great problem. One of my favorites for sure. But is this really an IMO question? It seems a little too easy.
Having 10 consecutive divisors doesn't imply that 11 is a divisor. The extra prime divisor comes as a result of the minimization requirement.
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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"?
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...
1 is divisor or not... because in 10 consecutive no need of 11
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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!!
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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
Why must you allocate another number? How do you know that?
nice problem just couldn't crack 32 and got 11^2 in place of it
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 1 0 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 1 0 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 2 5 2 0 ( h a s 1 0 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 − 1 0 ) 2 5 2 0 = 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 = 1 4 4 − − ( I ) F r o m ( i ) : N o o f d i v i s o r s = 1 4 4 = 3 × 3 × 4 × 4 W e c a n m u l t i p l y 2 5 2 0 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 ) = 1 4 4 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 = 2 6 2 4 0 0 ( w i t h 1 4 4 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 2 5 2 0 w i t h 3 5 2 . U s e o n e m o r e p r i m e 2 5 × 3 2 × 5 × 7 × 1 1 = 1 1 0 8 8 0 ( w i t h 1 4 4 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 4 4 L e a s t P o s s i b l e X = 1 1 0 8 8 0 ( w i t h 1 4 4 d i v i s o r s )
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Among ten consecutive integers that divide n, there must exist numbers divisible by 2 3 , 3 2 , 5 , 7 , 1 1 (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 , where α 1 ≥ 3 ; α 2 ≥ 2 ; α 3 ≥ α 4 ≥ a 5 ≥ 1 .
Since n has ( α 1 + 1 ) ( α 2 + 1 ) ( α 3 + 1 ) . . . distinct factors, and ( α 1 + 1 ) ( α 2 + 1 ) ( α 3 + 1 ) ( α 4 + 1 ) ≥ 4 8 , we must have ( α 5 + 1 ) ⋅ ⋅ ⋅ ≤ 3 . Hence at most one a j > 4 , Is positive, and in the minimal n this must be a 5 .
Checking through logic and some bashing, the possible combinations satisfying ( α 1 + 1 ) ( α 2 + 1 ) . . . ( α 5 + 1 ) = 1 4 4 one finds that the minimal n is 2 5 ⋅ 3 2 ⋅ 5 ⋅ 7 ⋅ 1 1 = 1 1 0 8 8 0 .