# discrete mathematics

1. prove by mathematical induction that: 1+2^n<=3^n for all n>=1.

2. give an example or prove that there are none.
a) a simple graph with degrees 1,2,2,3.
b) a simple graph with degrees 2,3,4,4,4.
c) a simple graph with degrees 1,1,2,4.
kaufmed

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Lucho_Nevara

for question 1, i went this far:

for n=1, 1+2^1<=3^1 is true
suppose 1+2^k<=3^k and let's show that 1+2^(k+1)<=3^(k+1) (this is where i'm stranded and can't move forward).

for question 2,
a) represent 4 nodes with the associations
b)there is no example because the sum of the degrees 2+3+4+4+4=17 is odd.
c)there is no example because the highest degree for a 4-node simple graph is 3.

For question 1:

Suppose 1 + 2^k <= 3^k for an k >= 1

you may try multiplying both side with (1+2) and see what happens.

it does not help me. suggest something different please.
Peter Kwan

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Upon expanding, I would have:

1+2^k+2+2^(k+1) <= 3^(k+1)

=> 1+2^(k+1)+2+2^k <= 3^(k+1)

we know that 1+2^(k+1) <= 1+2^(k+1)+2+2^k, therefore we have: 1+2^(k+1) <= 3^(k+1).
this completes the mathematical induction. Am I right?
Spot on, just need to complete with check of initial value (e.g. 2)