# DivisibilityWatch

#1
Does , where a, b and c are known, , and , ever have any solutions when ?

For example, could I infer that can only be true for or not? (Or is it even true?)
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9 years ago
#2
Well, I would first deduce that divides . But , so we conclude that , since there are no multiples of between 0 and . But , so .
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#3
Thank you, that completes my rather shoddy proof, but is it true in general?
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9 years ago
#4
Somehow I've made it through infant school, primary school, secondary school and up to my A2s without knowing how to do division on paper... so looking at that makes me brick myself
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9 years ago
#5
(Original post by rupertj)
Does , where a, b and c are known, , and , ever have any solutions when ?
n + 8 | 2n + 4 (a = 8, b = 2, c = 4, so gcd(b, a-c) = 2) has a solution n = 4.
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#6
(Original post by generalebriety)
n + 8 | 2n + 4 (a = 8, b = 2, c = 4, so gcd(b, a-c) = 2) has a solution n = 4.
Apologies, I meant , rather than 'is not equal to'.

I've edited it now.
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9 years ago
#7
(Original post by rupertj)
Apologies, I meant , rather than 'is not equal to'.

I've edited it now.
Ok. n+7|3n+2 (a=7, b=3, c=5, so gcd(b, a-c) = 1) has solution n=12.
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#8
(Original post by generalebriety)
Ok. n+7|3n+2 (a=7, b=3, c=5, so gcd(b, a-c) = 1) has solution n=12.
Okay, thanks. I'm so stupid.
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9 years ago
#9
(Original post by rupertj)
Okay, thanks. I'm so stupid.
In fact, here's something a bit stronger. Suppose we want to solve n+a | bn + (a-c). Then one solution obviously occurs if (n+a) divides (bn + (a-c)) precisely (b-1) times. That is:

(b-1)(n+a) = bn + a - c
i.e.
(b-1)n + a(b-1) = bn + a - c
and so
n = a(b-2) + c

which is always a positive integer if b >= 2.
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