Master theorem

2 Asymptotic growth

Definition 30
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\(f\) is asymptotically bounded above by \(g\) if there exists a \(k {\gt} 0\) such that \(f\) is asymptotically less than \(k*g\).

Definition 31
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\(f\) is asymptotically bounded below by \(g\) if there exists \(k {\gt} 0\) such that \(f\) is asymptotically greater than \(k*g\).

Definition 32
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\(f\) is asymptotically bounded by \(g\) if \(f\) is asymptotically bounded above and below by \(g\).

Definition 33
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\(f\) is asymptotically dominated by \(g\) if for all \(k {\gt} 0\) \(f\) is asymptotically less than \(k*g\).

Definition 34
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\(f\) asymptotically dominates \(g\) if for all \(k {\gt} 0\) \((x)\) is asymptotically greater than \(k*g\).

Lemma 35

If \(f\) is dominated by \(g\), then it’s also bounded above by \(g\).

Proof

The definitions of \(f\) being dominated and bounded above by \(g\) only differ in the quantifier before \(k\) at the very start (universal for the hypothesis, existential for the goal), so it suffices to use any positive value for \(k\). We can use \(1\). The desired result then follows directly.

Lemma 36

If \(f\) dominates \(g\), then it’s bounded below by \(g\).

Proof

The proof is entirely analogous to the previous proof.

Lemma 37

\(f\) is asymptotically bounded above and below by \(g\) if and only if \(f\) is asymptotically bounded by \(g\).

Proof

Both directions follow directly from the definition of asymptotic boundedness.

Let \(g\) be asymptotically positive. Then \(f\) is not both asymptotically bounded below by \(g\) and asymptotically dominated by \(g\).

Proof

Suppose \(f\) is asymptotically bounded below by \(g\) and also asymptotically dominated by \(g\). We need to find a contradiction to prove the statement.

First, we claim that there exists \(x\_ 0\) such that for all \(x \ge x\_ 0\) we have \(f(x) \ge k \cdot g(x)\) for some \(k {\gt} 0\), \(g(x) \ge 0\) and \(f(x) \le k' \cdot g(x)\) for all \(k' {\gt} 0\). Each of the asymptotic assumption gives one such constant, so taking the maximum of all three gives the needed value.

For the contradiction, consider \(f(x) \le k' \cdot g(x)\) when \(k' = k / 2\). In this case, we have \(f(x) \le (k / 2) \cdot g(x)\), but we also have \(k \cdot g(x) \le f(x)\), leading by transitivity to \(k \cdot g(x) \le (k / 2) \cdot g(x)\), an obvious contradiction to the fact that \((k / 2) \cdot g(x) \le k \cdot g(x)\).

Let \(f\) be asymptotically positive. If \(f\) is asymptotically bounded below by \(g\), then \(f\) is not asymptotically dominated by \(g\).

Proof

This is a direct application of Lemma 38.

Let \(f\) be asymptotically positive. If \(f\) is asymptotically bounded below by \(g\), then \(f\) is not asymptotically dominated by \(g\).

Proof

This is a direct application of Lemma 38.

Let \(g\) be asymptotically positive. Then it does not hold that both \(f\) is asymptotically bounded above by \(g\) and \(f\) asymptotically dominate \(g\).

Proof

The proof is analogous to the proof of Lemma 38. This time, we set \(k' = k + 1\) and thus produce the false inequality \((k + 1) \cdot g(x) \le k \cdot g(x)\).

Let \(f\) be asymptotically positive. If \(f\) is bounded below by \(g\), then \(f\) does not dominate \(g\).

Proof

This is a direct application of Lemma 41.

Let \(f\) be asymptotically positive. If \(f\) is asymptotically bounded above by \(g\), then \(f\) does not asymptotically dominate \(g\).

Proof

This is a direct application of Lemma 41.

Let \(g\) be asymptotically positive. Then it is not true that both \(f\) is asymptotically dominated by \(g\) and that \(f\) dominates \(g\).

Proof

Suppose \(f\) both dominates \(g\) and is dominated by \(g\). Our goal is to find a contradiction. We have by definition the inequalities \(g(x) {\gt} 0\), \(f(x) \ge k\_ 1 \cdot g(x)\) and \(f(x) \le k\_ 2 \cdot g(x)\) for all \(k\_ 1 {\gt} 0\), \(k\_ 2 {\gt} 0\) and for all \(x \ge x\_ 0\) for some \(x\_ 0\). Note that we use the same constant \(x\_ 0\) for all inequalities with no loss of generality. In fact, we shall also use the same \(x\_ 0\) in the asymptotic positivity condition \(g(x) {\gt} 0\).

Fix \(k\_ 1 = 2\) and \(k\_ 2 = 1\) (generally, we only need \(k\_ 1 \ge k\_ 2\)), so we now have \(f(x) \ge 2 \cdot g(x)\) and \(f(x) \le g(x)\). From these inequalities, it immediately follows that \(2 \cdot g(x) \le g(x)\). However, since \(1 \le 2\) and \(g(x) {\gt} 0\), we have \(g(x) {\lt} 2 \cdot g(x)\). We thus have two contradicting inequalities, finishing the proof.

Let \(g\) be asymptotically positive. If \(f\) is asymptotically dominated by \(g\), then \(f\) does not asymptotically dominate \(g\).

Proof

This is a direct application of Lemma 44.

Let \(g\) be asymptotically positive. If \(f\) asymptotically dominates \(g\), then \(f\) is not asymptotically dominated by \(g\).

Proof

This is a direct application of Lemma 44.

2.1 Reflexivity

Lemma 47

\(f\) is asymptotically bounded by itself.

Proof

Proving asymptotic boundedness is equivalent to proving boundedness above and below. Both can be proved the same way - we choose \(1_K\) for \(K\) and \(1_R\) for \(N\), then the required asymptotic growth properties follow from definitions of identity elements for \(K\) and \(R\).

Lemma 48
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\(f\) is asymptotically bounded above by itself.

Proof

This follows directly from 47.

Lemma 49
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\(f\) is asymptotically bounded below by itself.

Proof

This follows directly from 47.

2.2 Transitivity

Lemma 50

If \(f\) is asymptotically bounded above by \(g\) and \(g\) is asymptotically bounded above by \(h\), then \(f\) is asymptotically bounded above by \(h\).

Proof

Let \(k\_ 1\) and \(k\_ 2\) be the constants such that \(f(x) \le k\_ 1 \cdot g(x)\) and \(g(x) \le k\_ 2 \cdot h(x)\) for sufficiently large \(x\). By multiplicativity and transitivity, we have \(f(x) \le k\_ 1 \cdot k\_ 2 \cdot h(x)\).

Lemma 51

If \(f\) is asymptotically bounded below by \(g\) and \(g\) is asymptotically bounded below by \(h\), then \(f\) is asymptotically bounded below by \(h\).

Proof

Let \(k\_ 1\) and \(k\_ 2\) be the constants such that \(f(x) \ge k\_ 1 \cdot g(x)\) and \(g(x) \ge k\_ 2 \cdot h(x)\) for sufficiently large \(x\). By multiplicativity and transitivity, we have \(f(x) \ge k\_ 1 \cdot k\_ 2 \cdot h(x)\).

Lemma 52
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If \(f\) is asymptotically bounded by \(g\) and \(g\) is asymptotically bounded by \(h\), then \(f\) is asymptotically bounded by \(h\).

Proof

A direct consequence of Lemma 50 and Lemma 51.

Lemma 53
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If \(f\) is asymptotically dominated by \(g\) and \(g\) is asymptotically dominated by \(h\), then \(f\) is asymptotically dominated by \(h\).

Proof

Let \(k {\gt} 0\). Then by first assumption, we have \(f(x) \le k \cdot g(x)\) for large \(x\). By the second assumption, we have \(g(x) \le h(x)\) for large \(x\). By multiplicativity and transitivity, we get \(f(x) \le k \cdot h(x)\).

Lemma 54
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If \(f\) asymptotically dominates \(g\) and \(g\) asymptotically dominates \(h\), then \(f\) asymptotically dominates \(h\).

Proof

Let \(k {\gt} 0\). Then by first assumption, we have \(f(x) \ge k \cdot g(x)\) for large \(x\). By the second assumption, we have \(g(x) \ge h(x)\) for large \(x\). By multiplicativity and transitivity, we get \(f(x) \ge k \cdot h(x)\).

2.3 Scalar multiplicativity

Lemma 55
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Let \(c {\gt} 0\). If \(f\) is bounded above by \(g\), then \(c \cdot f\) is also bounded above by \(g\).

Proof

Let \(k\) be the constant such that \(f\) is asymptotically less than \(k \cdot g\). By positive scalar multiplicativity of asymptotic inequality, \(c \cdot f\) is asymptotically less than \(c \cdot k \cdot g\). Since \(c \cdot k {\gt} 0\), this implies that \(c \cdot f\) is bounded above by \(g\).

Lemma 56
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Let \(c {\gt} 0\). If \(f\) is bounded below by \(g\), then \(c \cdot f\) is also bounded below by \(g\).

Proof

Let \(k\) be the constant such that \(f\) is asymptotically greater than \(k \cdot g\). By positive scalar multiplicativity of asymptotic inequality, \(c \cdot f\) is asymptotically greater than \(c \cdot k \cdot g\). Since \(c \cdot k {\gt} 0\), this implies that \(c \cdot f\) is bounded below by \(g\).

Lemma 57
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Let \(c {\gt} 0\). If \(f\) is bounded by \(g\), then \(c \cdot f\) is also bounded by \(g\).

Proof

Above boundedness is exactly Lemma 55 and below boundedness is exactly Lemma 56.

Lemma 58
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Let \(c {\lt} 0\). If \(f\) is bounded above by \(g\), then \(c \cdot f\) is bounded above by \(-g\).

Proof

Let \(k\) be the constant such that \(f\) is asymptotically less than \(k \cdot g\). By positive scalar multiplicativity of asymptotic inequality, \(-c \cdot f\) is asymptotically less than \(-c \cdot k \cdot g\). Since \(-c \cdot k {\gt} 0\), this implies that \(c \cdot f\) is bounded above by \(-g\).

Lemma 59
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Let \(c {\lt} 0\). If \(f\) is bounded below by \(g\), then \(c \cdot f\) is bounded below by \(-g\).

Proof

Let \(k\) be the constant such that \(f\) is asymptotically greater than \(k \cdot g\). By positive scalar multiplicativity of asymptotic inequality, \(-c \cdot f\) is asymptotically greater than \(-c \cdot k \cdot g\). Since \(-c \cdot k {\gt} 0\), this implies that \(c \cdot f\) is bounded below by \(-g\).

Lemma 60
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Let \(c {\lt} 0\). If \(f\) is bounded by \(g\), then \(c \cdot f\) is bounded by \(-g\).

Proof

Above boundedness is exactly Lemma 58 and below boundedness is exactly Lemma 59.

2.4 Additivity

Lemma 61
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Let \(f\_ 1\) and \(f\_ 2\) be bounded above by \(g\). Then \(f\_ 1 + f\_ 2\) is also bounded above by \(g\).

Proof

Let \(k\_ 1\) and \(k\_ 2\) be the constants such that \(f\_ 1\) is asymptotically less than \(k\_ 1 \cdot g\) and \(f\_ 2\) is asymptotically less than \(k\_ 2 \cdot g\). By additivity of asymptotic inequality, \(f\_ 1 + f\_ 2\) is asymptotically less than \((k\_ 1 * k\_ 2) \cdot g\). It directly follows that \(f\_ 1 + f\_ 2\) is asymptotically bounded above by \(g\).

Lemma 62
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Let \(f\_ 1\) and \(f\_ 2\) be bounded below by \(g\). Then \(f\_ 1 + f\_ 2\) is also bounded below by \(g\).

Proof

Let \(k\_ 1\) and \(k\_ 2\) be the constants such that \(f\_ 1\) is asymptotically greater than \(k\_ 1 \cdot g\) and \(f\_ 2\) is asymptotically greater than \(k\_ 2 \cdot g\). By additivity of asymptotic inequality, \(f\_ 1 + f\_ 2\) is asymptotically greater than \((k\_ 1 * k\_ 2) \cdot g\). It directly follows that \(f\_ 1 + f\_ 2\) is asymptotically bounded below by \(g\).

Lemma 63
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Let \(f\_ 1\) and \(f\_ 2\) be bounded by \(g\). Then \(f\_ 1 + f\_ 2\) is also bounded by \(g\).

Proof

This is proved directly by Lemma 61 and Lemma 62.

Lemma 64

Let \(f\_ 1\) be bounded below by \(g\) and let \(f\_ 2\) be asymptotically positive. Then \(f\_ 1 + f\_ 2\) is bounded below by \(g\).

Proof

This property immediately follows from Lemma 22.

Lemma 65

Let \(f\_ 1\) be bounded above by \(g\) and let \(f\_ 2\) be asymptotically negative. Then \(f\_ 1 + f\_ 2\) is bounded above by \(g\).

Proof

This property immediately follows from Lemma 23.

Let \(f\_ 1\) be bounded by \(g\). Let also \(f\_ 2\) be asymptotically positive and bounded above by \(g\). Then \(f\_ 1 + f\_ 2\) is bounded by \(g\).

Proof

This property immediately follows from Lemma 61 and Lemma 64.

Let \(f\_ 1\) be bounded by \(g\). Let also \(f\_ 2\) be asymptotically negative and bounded below by \(g\). Then \(f\_ 1 + f\_ 2\) is bounded by \(g\).

Proof

This property immediately follows from Lemma 62 and Lemma 65.

Let \(f\_ 1\) be bounded by \(g\). Let also \(f\_ 2\) be asymptotically positive and let \(f\_ 2\) be dominated by \(g\). Then \(f\_ 1 + f\_ 2\) is bounded by \(g\).

Proof

We prove this with an application of Lemma 66 on Lemma 35.

Let \(f\_ 1\) be bounded by \(g\). Let also \(f\_ 2\) be asymptotically negative and let \(f\_ 2\) dominate \(g\). Then \(f\_ 1 + f\_ 2\) is bounded by \(g\).

Proof

We prove this with an application of Lemma 67 on Lemma 36.

2.5 Multiplicativity

Lemma 70

Let \(f\_ 1\) and \(f\_ 2\) be asymptotically nonnegative functions such that \(f\_ 1\) is asymptotically bounded above by \(g\_ 1\) and \(f\_ 2\) is asymptotically bounded above by \(g\_ 2\). Then \(f\_ 1 * f\_ 2\) is asymptotically bounded above by \(g\_ 1 * g\_ 2\).

Proof

Let \(k\_ 1\) and \(k\_ 2\) be the constants such that \(f\_ 1\) is asymptotically less than \(g\_ 1\) and \(f\_ 2\) is asymptotically less than \(g\_ 2\). By multiplicativity of asymptotic inequality, \(f\_ 1 * f\_ 2\) is asymptotically less than \(k\_ 1 \cdot g\_ 1 * k\_ 2 \cdot g\_ 2\), which is equivalent to asymptotic above boundedness of \(f\_ 1 * f\_ 2\) by \(g\_ 1 * g\_ 2\).

Lemma 71

Let \(f\_ 1\) and \(f\_ 2\) be asymptotically nonpositive functions such that \(f\_ 1\) is asymptotically bounded below by \(g\_ 1\) and \(f\_ 2\) is asymptotically bounded below by \(g\_ 2\). Then \(f\_ 1 * f\_ 2\) is asymptotically bounded below by \(g\_ 1 * g\_ 2\).

Proof

Let \(k\_ 1\) and \(k\_ 2\) be the constants such that \(f\_ 1\) is asymptotically greaterthan \(g\_ 1\) and \(f\_ 2\) is asymptotically greater than \(g\_ 2\). By multiplicativity of asymptotic inequality, \(f\_ 1 * f\_ 2\) is asymptotically less than \(k\_ 1 \cdot g\_ 1 * k\_ 2 \cdot g\_ 2\), which is equivalent to asymptotic below boundedness of \(f\_ 1 * f\_ 2\) by \(g\_ 1 * g\_ 2\).