A number theory problem by K. J. W.

Find the largest positive integer n so that n < 100 and

n ! + ( n + 1 ) ! + ( n + 2 ) ! = q 2 n!+\left( n+1 \right) !+\left( n+2 \right) !={ q }^{ 2 }

where q is a positive integer.


The answer is 1.

This section requires Javascript.
You are seeing this because something didn't load right. We suggest you, (a) try refreshing the page, (b) enabling javascript if it is disabled on your browser and, finally, (c) loading the non-javascript version of this page . We're sorry about the hassle.

1 solution

K. J. W.
Jul 11, 2014

N o t e t h a t n ! + ( n + 1 ) ! + ( n + 2 ) ! = n ! + n ! ( n + 1 ) + n ! ( n + 1 ) ( n + 2 ) = n ! ( 1 + n + 1 + n 2 + 3 n + 2 ) = n ! ( n 2 + 4 n + 4 ) = n ! ( n + 2 ) 2 T h e r e f o r e n ! m u s t b e a s q u a r e n u m b e r . N o t e t h a t i f p ! x ! < ( 2 p ) ! w h e r e p i s a p r i m e n u m b e r , x ! i s d i v i s i b l e b y p b u t n o t b y p 2 . T h u s x ! c a n n o t b e a s q u a r e n u m b e r . F o r t h e c a s e p = 53 , x c a n b e a n y n u m b e r f r o m 53 t o 100. T h u s n c a n n o t b e f r o m 53 t o 100. F o r t h e c a s e p = 29 , x c a n b e a n y n u m b e r f r o m 29 t o 52. T h u s n c a n n o t b e f r o m 29 t o 100. F o r t h e c a s e p = 17 , x c a n b e a n y n u m b e r f r o m 17 t o 28. T h u s n c a n n o t b e f r o m 17 t o 100. F o r t h e c a s e p = 11 , x c a n b e a n y n u m b e r f r o m 11 t o 16. T h u s n c a n n o t b e f r o m 11 t o 100. F o r t h e c a s e p = 7 , x c a n b e a n y n u m b e r f r o m 7 t o 10. T h u s n c a n n o t b e f r o m 7 t o 100. W e a r e l e f t w i t h n = 1 , 2 , 3 , 4 , 5 , 6 a n d i t c a n b e e a s i l y c h e c k e d t h a t o f t h e s e , o n l y n = 1 s a t i s f i e s t h e a b o v e c o n d i t i o n s . T h e r e f o r e t h e l a r g e s t ( a n d o n l y ) s o l u t i o n i s n = 1. Note\quad that\quad n!+(n+1)!+(n+2)!\\ \\ =\quad n!+n!(n+1)+n!(n+1)(n+2)\\ =\quad n!(1+n+1+{ n }^{ 2 }+3n+2)\\ =\quad n!({ n }^{ 2 }+4n+4)\\ =\quad n!{ (n+2) }^{ 2 }\\ \\ Therefore\quad n!\quad must\quad be\quad a\quad square\quad number.\\ \\ Note\quad that\quad if\quad p!\quad \le \quad x!\quad <\quad (2p)!\quad where\quad p\quad is\quad a\quad prime\quad number,\\ x!\quad is\quad divisible\quad by\quad p\quad but\quad not\quad by\quad { p }^{ 2 }.\quad Thus\quad x!\quad cannot\quad be\quad a\quad square\quad number.\\ \\ For\quad the\quad case\quad p=53,\quad x\quad can\quad be\quad any\quad number\quad from\quad 53\quad to\quad 100.\quad Thus\quad n\quad cannot\quad be\quad from\quad 53\quad to\quad 100.\\ \\ For\quad the\quad case\quad p=29,\quad x\quad can\quad be\quad any\quad number\quad from\quad 29\quad to\quad 52.\quad Thus\quad n\quad cannot\quad be\quad from\quad 29\quad to\quad 100.\\ \\ For\quad the\quad case\quad p=17,\quad x\quad can\quad be\quad any\quad number\quad from\quad 17\quad to\quad 28.\quad Thus\quad n\quad cannot\quad be\quad from\quad 17\quad to\quad 100.\\ \\ For\quad the\quad case\quad p=11,\quad x\quad can\quad be\quad any\quad number\quad from\quad 11\quad to\quad 16.\quad Thus\quad n\quad cannot\quad be\quad from\quad 11\quad to\quad 100.\\ \\ For\quad the\quad case\quad p=7,\quad x\quad can\quad be\quad any\quad number\quad from\quad 7\quad to\quad 10.\quad Thus\quad n\quad cannot\quad be\quad from\quad 7\quad to\quad 100.\\ \\ We\quad are\quad left\quad with\quad n=1,2,3,4,5,6\quad and\quad it\quad can\quad be\quad easily\quad checked\quad that\quad of\quad these,\quad only\quad n=1\quad satisfies\quad the\quad above\quad conditions.\\ \\ Therefore\quad the\quad largest\quad (and\quad only)\quad solution\quad is\quad n=1.\\

0 pending reports

×

Problem Loading...

Note Loading...

Set Loading...