Divisibility

If a and b are positive integers such that for all positive integers k, ak| bk+1, then prove that a|b

11 Answers

341
Hari Shankar ·

how do you get a^k|\delta^{k+1}?

9
Celestine preetham ·

sorry i was goin nuts now edited

341
Hari Shankar ·

as k→∞ LHS → aλk+1 - how do you arrive at that?

9
Celestine preetham ·

sorry sir for repeatedly posting wrong solutions :(

9
Celestine preetham ·

hope this is cent percent rite

(aλ + δ )k+1 = ak.β

for k=1 a|δ2

for k=2 a2|δ3
( as (aλ+δ)3 /a2 = aμ + ρ +ξδ2/a + δ3/a2 )

similarly for any k

ak| δk+1

now as δk+1 = β. ak and δ<a

for k→∞ , β=0 → δ=0

hence proved

341
Hari Shankar ·

somethings need to be cleared up. β is not a constant. It is a function of k. So as k→∞, β(k)→0.

So, now how do we conclude that δ = 0?

9
Celestine preetham ·

δ is a constant

so if δ =0 when k→∞

its always 0 !

341
Hari Shankar ·

See β→0, but ak→∞. So what tells you that δ=0?

9
Celestine preetham ·

β→0 implies β=0 as β is integer

im sure this time its flawless :P

341
Hari Shankar ·

that looks alright(phew!)

there another way - when we say p|q what we are saying is that if r is a prime that occurs in p, it occurs to a higher power in q. This power is denoted by ordr(p)

Now, if for some prime p, we have ordp(a)>ordp(b)

Let ordp(a) = m and ordp(b) = n

Now for all k>n/(m-n) we will have ordp(ak) > ordp(bk+1)

which means ak does not divide bk+1 for k>n/(m-n)

9
Celestine preetham ·

why the (phew!) ?

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