The Runtime Complexity (innermost) of the given CpxTRS could be proven to be BOUNDS(n^1, INF).

(0) Obligation:

Runtime Complexity TRS:
The TRS R consists of the following rules:

a(b(x)) → b(b(a(a(x))))

Rewrite Strategy: INNERMOST

(1) RenamingProof (EQUIVALENT transformation)

Renamed function symbols to avoid clashes with predefined symbol.

(2) Obligation:

Runtime Complexity Relative TRS:
The TRS R consists of the following rules:

a(b(x)) → b(b(a(a(x))))

S is empty.
Rewrite Strategy: INNERMOST

(3) TypeInferenceProof (BOTH BOUNDS(ID, ID) transformation)

Infered types.

(4) Obligation:

Innermost TRS:
Rules:
a(b(x)) → b(b(a(a(x))))

Types:
a :: b → b
b :: b → b
hole_b1_0 :: b
gen_b2_0 :: Nat → b

(5) OrderProof (LOWER BOUND(ID) transformation)

Heuristically decided to analyse the following defined symbols:
a

(6) Obligation:

Innermost TRS:
Rules:
a(b(x)) → b(b(a(a(x))))

Types:
a :: b → b
b :: b → b
hole_b1_0 :: b
gen_b2_0 :: Nat → b

Generator Equations:
gen_b2_0(0) ⇔ hole_b1_0
gen_b2_0(+(x, 1)) ⇔ b(gen_b2_0(x))

The following defined symbols remain to be analysed:
a

(7) RewriteLemmaProof (LOWER BOUND(ID) transformation)

Proved the following rewrite lemma:
a(gen_b2_0(+(1, n4_0))) → *3_0, rt ∈ Ω(n4₀)

Induction Base:
a(gen_b2_0(+(1, 0)))

Induction Step:
a(gen_b2_0(+(1, +(n4_0, 1)))) →_R^Ω(1)
b(b(a(a(gen_b2_0(+(1, n4_0)))))) →_IH
b(b(a(*3_0)))

We have rt ∈ Ω(n¹) and sz ∈ O(n). Thus, we have irc_R ∈ Ω(n).

(8) Complex Obligation (BEST)

(9) Obligation:

Innermost TRS:
Rules:
a(b(x)) → b(b(a(a(x))))

Types:
a :: b → b
b :: b → b
hole_b1_0 :: b
gen_b2_0 :: Nat → b

Lemmas:
a(gen_b2_0(+(1, n4_0))) → *3_0, rt ∈ Ω(n4₀)

Generator Equations:
gen_b2_0(0) ⇔ hole_b1_0
gen_b2_0(+(x, 1)) ⇔ b(gen_b2_0(x))

No more defined symbols left to analyse.

(10) LowerBoundsProof (EQUIVALENT transformation)

The lowerbound Ω(n¹) was proven with the following lemma:
a(gen_b2_0(+(1, n4_0))) → *3_0, rt ∈ Ω(n4₀)

(11) BOUNDS(n^1, INF)

(12) Obligation:

Innermost TRS:
Rules:
a(b(x)) → b(b(a(a(x))))

Types:
a :: b → b
b :: b → b
hole_b1_0 :: b
gen_b2_0 :: Nat → b

Lemmas:
a(gen_b2_0(+(1, n4_0))) → *3_0, rt ∈ Ω(n4₀)

Generator Equations:
gen_b2_0(0) ⇔ hole_b1_0
gen_b2_0(+(x, 1)) ⇔ b(gen_b2_0(x))

No more defined symbols left to analyse.

(13) LowerBoundsProof (EQUIVALENT transformation)

The lowerbound Ω(n¹) was proven with the following lemma:
a(gen_b2_0(+(1, n4_0))) → *3_0, rt ∈ Ω(n4₀)

(14) BOUNDS(n^1, INF)