- #1
Seydlitz
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Homework Statement
I'm still not confident enough to answer this.
Prove that if a set ##A## of natural numbers contains ##n_{0}## and contains ##k + 1## whenever it contains ##k##, then ##A## contains all natural numbers ##> n_{0}##.
Homework Equations
Proof by induction methods
The Attempt at a Solution
I'm mirroring the similar method in page.23 of Spivak's calculus book.
Suppose ##A## has ##n_{0}##, let B be a set of all ##l## such that, ##n_{0}+l## is in ##A##. Then ##1## is in ##B##, so does ##l+1## assuming ##l## is in ##B##. This show that ##B## is a set of all natural numbers, and it is in fact also contained in ##A##. So there exist all natural number which is larger than ##n_{0}##.
Practically it should work like this right?
##A=\{1,2,3,4,5,6,7...n\}##
Say ##n_{0}## is = ##4##. Then there exist also in ##A## all integers which is larger than ##4##.
When I say let B a a set of all ##l## such that, ##n_{0}+l## is in ##A##. Then ##B## looks like this. ##B=\{1,2,3,4,5,6,7,8..l\}## so that in ##A=\{1...5, 6, 7, 8, 9, 10, 11, 12, 13...n_{0}+l\}##
Though my question is, why we need to create set ##B## in this case, to prove what set ##A## contains. Is it related somehow to set theory, where if you want to show two sets are equal, you have to make show that they are the subset of each other? Can you guys please enlighten me on this point.
(This is like magic, by just writing the opening post, I think I understand better here than few minutes ago...)