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:

plus(0, y) → y
plus(s(x), y) → s(plus(x, y))
lt(0, s(y)) → true
lt(x, 0) → false
lt(s(x), s(y)) → lt(x, y)
fib(x) → fibiter(x, 0, 0, s(0))
fibiter(b, c, x, y) → if(lt(c, b), b, c, x, y)
if(false, b, c, x, y) → x
if(true, b, c, x, y) → fibiter(b, s(c), y, plus(x, y))

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:

plus(0', y) → y
plus(s(x), y) → s(plus(x, y))
lt(0', s(y)) → true
lt(x, 0') → false
lt(s(x), s(y)) → lt(x, y)
fib(x) → fibiter(x, 0', 0', s(0'))
fibiter(b, c, x, y) → if(lt(c, b), b, c, x, y)
if(false, b, c, x, y) → x
if(true, b, c, x, y) → fibiter(b, s(c), y, plus(x, y))

S is empty.
Rewrite Strategy: INNERMOST

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

Infered types.

(4) Obligation:

Innermost TRS:
Rules:
plus(0', y) → y
plus(s(x), y) → s(plus(x, y))
lt(0', s(y)) → true
lt(x, 0') → false
lt(s(x), s(y)) → lt(x, y)
fib(x) → fibiter(x, 0', 0', s(0'))
fibiter(b, c, x, y) → if(lt(c, b), b, c, x, y)
if(false, b, c, x, y) → x
if(true, b, c, x, y) → fibiter(b, s(c), y, plus(x, y))

Types:
plus :: 0':s → 0':s → 0':s
0' :: 0':s
s :: 0':s → 0':s
lt :: 0':s → 0':s → true:false
true :: true:false
false :: true:false
fib :: 0':s → 0':s
fibiter :: 0':s → 0':s → 0':s → 0':s → 0':s
if :: true:false → 0':s → 0':s → 0':s → 0':s → 0':s
hole_0':s1_0 :: 0':s
hole_true:false2_0 :: true:false
gen_0':s3_0 :: Nat → 0':s

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

Heuristically decided to analyse the following defined symbols:
plus, lt, fibiter

They will be analysed ascendingly in the following order:
plus < fibiter
lt < fibiter

(6) Obligation:

Innermost TRS:
Rules:
plus(0', y) → y
plus(s(x), y) → s(plus(x, y))
lt(0', s(y)) → true
lt(x, 0') → false
lt(s(x), s(y)) → lt(x, y)
fib(x) → fibiter(x, 0', 0', s(0'))
fibiter(b, c, x, y) → if(lt(c, b), b, c, x, y)
if(false, b, c, x, y) → x
if(true, b, c, x, y) → fibiter(b, s(c), y, plus(x, y))

Types:
plus :: 0':s → 0':s → 0':s
0' :: 0':s
s :: 0':s → 0':s
lt :: 0':s → 0':s → true:false
true :: true:false
false :: true:false
fib :: 0':s → 0':s
fibiter :: 0':s → 0':s → 0':s → 0':s → 0':s
if :: true:false → 0':s → 0':s → 0':s → 0':s → 0':s
hole_0':s1_0 :: 0':s
hole_true:false2_0 :: true:false
gen_0':s3_0 :: Nat → 0':s

Generator Equations:
gen_0':s3_0(0) ⇔ 0'
gen_0':s3_0(+(x, 1)) ⇔ s(gen_0':s3_0(x))

The following defined symbols remain to be analysed:
plus, lt, fibiter

They will be analysed ascendingly in the following order:
plus < fibiter
lt < fibiter

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

Proved the following rewrite lemma:
plus(gen_0':s3_0(n5_0), gen_0':s3_0(b)) → gen_0':s3_0(+(n5_0, b)), rt ∈ Ω(1 + n5₀)

Induction Base:
plus(gen_0':s3_0(0), gen_0':s3_0(b)) →_R^Ω(1)
gen_0':s3_0(b)

Induction Step:
plus(gen_0':s3_0(+(n5_0, 1)), gen_0':s3_0(b)) →_R^Ω(1)
s(plus(gen_0':s3_0(n5_0), gen_0':s3_0(b))) →_IH
s(gen_0':s3_0(+(b, c6_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:
plus(0', y) → y
plus(s(x), y) → s(plus(x, y))
lt(0', s(y)) → true
lt(x, 0') → false
lt(s(x), s(y)) → lt(x, y)
fib(x) → fibiter(x, 0', 0', s(0'))
fibiter(b, c, x, y) → if(lt(c, b), b, c, x, y)
if(false, b, c, x, y) → x
if(true, b, c, x, y) → fibiter(b, s(c), y, plus(x, y))

Types:
plus :: 0':s → 0':s → 0':s
0' :: 0':s
s :: 0':s → 0':s
lt :: 0':s → 0':s → true:false
true :: true:false
false :: true:false
fib :: 0':s → 0':s
fibiter :: 0':s → 0':s → 0':s → 0':s → 0':s
if :: true:false → 0':s → 0':s → 0':s → 0':s → 0':s
hole_0':s1_0 :: 0':s
hole_true:false2_0 :: true:false
gen_0':s3_0 :: Nat → 0':s

Lemmas:
plus(gen_0':s3_0(n5_0), gen_0':s3_0(b)) → gen_0':s3_0(+(n5_0, b)), rt ∈ Ω(1 + n5₀)

Generator Equations:
gen_0':s3_0(0) ⇔ 0'
gen_0':s3_0(+(x, 1)) ⇔ s(gen_0':s3_0(x))

The following defined symbols remain to be analysed:
lt, fibiter

They will be analysed ascendingly in the following order:
lt < fibiter

(10) RewriteLemmaProof (LOWER BOUND(ID) transformation)

Proved the following rewrite lemma:
lt(gen_0':s3_0(n554_0), gen_0':s3_0(+(1, n554_0))) → true, rt ∈ Ω(1 + n554₀)

Induction Base:
lt(gen_0':s3_0(0), gen_0':s3_0(+(1, 0))) →_R^Ω(1)
true

Induction Step:
lt(gen_0':s3_0(+(n554_0, 1)), gen_0':s3_0(+(1, +(n554_0, 1)))) →_R^Ω(1)
lt(gen_0':s3_0(n554_0), gen_0':s3_0(+(1, n554_0))) →_IH
true

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

(11) Complex Obligation (BEST)

(12) Obligation:

Innermost TRS:
Rules:
plus(0', y) → y
plus(s(x), y) → s(plus(x, y))
lt(0', s(y)) → true
lt(x, 0') → false
lt(s(x), s(y)) → lt(x, y)
fib(x) → fibiter(x, 0', 0', s(0'))
fibiter(b, c, x, y) → if(lt(c, b), b, c, x, y)
if(false, b, c, x, y) → x
if(true, b, c, x, y) → fibiter(b, s(c), y, plus(x, y))

Types:
plus :: 0':s → 0':s → 0':s
0' :: 0':s
s :: 0':s → 0':s
lt :: 0':s → 0':s → true:false
true :: true:false
false :: true:false
fib :: 0':s → 0':s
fibiter :: 0':s → 0':s → 0':s → 0':s → 0':s
if :: true:false → 0':s → 0':s → 0':s → 0':s → 0':s
hole_0':s1_0 :: 0':s
hole_true:false2_0 :: true:false
gen_0':s3_0 :: Nat → 0':s

Lemmas:
plus(gen_0':s3_0(n5_0), gen_0':s3_0(b)) → gen_0':s3_0(+(n5_0, b)), rt ∈ Ω(1 + n5₀)
lt(gen_0':s3_0(n554_0), gen_0':s3_0(+(1, n554_0))) → true, rt ∈ Ω(1 + n554₀)

Generator Equations:
gen_0':s3_0(0) ⇔ 0'
gen_0':s3_0(+(x, 1)) ⇔ s(gen_0':s3_0(x))

The following defined symbols remain to be analysed:
fibiter

(13) NoRewriteLemmaProof (LOWER BOUND(ID) transformation)

Could not prove a rewrite lemma for the defined symbol fibiter.

(14) Obligation:

Innermost TRS:
Rules:
plus(0', y) → y
plus(s(x), y) → s(plus(x, y))
lt(0', s(y)) → true
lt(x, 0') → false
lt(s(x), s(y)) → lt(x, y)
fib(x) → fibiter(x, 0', 0', s(0'))
fibiter(b, c, x, y) → if(lt(c, b), b, c, x, y)
if(false, b, c, x, y) → x
if(true, b, c, x, y) → fibiter(b, s(c), y, plus(x, y))

Types:
plus :: 0':s → 0':s → 0':s
0' :: 0':s
s :: 0':s → 0':s
lt :: 0':s → 0':s → true:false
true :: true:false
false :: true:false
fib :: 0':s → 0':s
fibiter :: 0':s → 0':s → 0':s → 0':s → 0':s
if :: true:false → 0':s → 0':s → 0':s → 0':s → 0':s
hole_0':s1_0 :: 0':s
hole_true:false2_0 :: true:false
gen_0':s3_0 :: Nat → 0':s

Lemmas:
plus(gen_0':s3_0(n5_0), gen_0':s3_0(b)) → gen_0':s3_0(+(n5_0, b)), rt ∈ Ω(1 + n5₀)
lt(gen_0':s3_0(n554_0), gen_0':s3_0(+(1, n554_0))) → true, rt ∈ Ω(1 + n554₀)

Generator Equations:
gen_0':s3_0(0) ⇔ 0'
gen_0':s3_0(+(x, 1)) ⇔ s(gen_0':s3_0(x))

No more defined symbols left to analyse.

(15) LowerBoundsProof (EQUIVALENT transformation)

The lowerbound Ω(n¹) was proven with the following lemma:
plus(gen_0':s3_0(n5_0), gen_0':s3_0(b)) → gen_0':s3_0(+(n5_0, b)), rt ∈ Ω(1 + n5₀)

(16) BOUNDS(n^1, INF)

(17) Obligation:

Innermost TRS:
Rules:
plus(0', y) → y
plus(s(x), y) → s(plus(x, y))
lt(0', s(y)) → true
lt(x, 0') → false
lt(s(x), s(y)) → lt(x, y)
fib(x) → fibiter(x, 0', 0', s(0'))
fibiter(b, c, x, y) → if(lt(c, b), b, c, x, y)
if(false, b, c, x, y) → x
if(true, b, c, x, y) → fibiter(b, s(c), y, plus(x, y))

Types:
plus :: 0':s → 0':s → 0':s
0' :: 0':s
s :: 0':s → 0':s
lt :: 0':s → 0':s → true:false
true :: true:false
false :: true:false
fib :: 0':s → 0':s
fibiter :: 0':s → 0':s → 0':s → 0':s → 0':s
if :: true:false → 0':s → 0':s → 0':s → 0':s → 0':s
hole_0':s1_0 :: 0':s
hole_true:false2_0 :: true:false
gen_0':s3_0 :: Nat → 0':s

Lemmas:
plus(gen_0':s3_0(n5_0), gen_0':s3_0(b)) → gen_0':s3_0(+(n5_0, b)), rt ∈ Ω(1 + n5₀)
lt(gen_0':s3_0(n554_0), gen_0':s3_0(+(1, n554_0))) → true, rt ∈ Ω(1 + n554₀)

Generator Equations:
gen_0':s3_0(0) ⇔ 0'
gen_0':s3_0(+(x, 1)) ⇔ s(gen_0':s3_0(x))

No more defined symbols left to analyse.

(18) LowerBoundsProof (EQUIVALENT transformation)

The lowerbound Ω(n¹) was proven with the following lemma:
plus(gen_0':s3_0(n5_0), gen_0':s3_0(b)) → gen_0':s3_0(+(n5_0, b)), rt ∈ Ω(1 + n5₀)

(19) BOUNDS(n^1, INF)

(20) Obligation:

Innermost TRS:
Rules:
plus(0', y) → y
plus(s(x), y) → s(plus(x, y))
lt(0', s(y)) → true
lt(x, 0') → false
lt(s(x), s(y)) → lt(x, y)
fib(x) → fibiter(x, 0', 0', s(0'))
fibiter(b, c, x, y) → if(lt(c, b), b, c, x, y)
if(false, b, c, x, y) → x
if(true, b, c, x, y) → fibiter(b, s(c), y, plus(x, y))

Types:
plus :: 0':s → 0':s → 0':s
0' :: 0':s
s :: 0':s → 0':s
lt :: 0':s → 0':s → true:false
true :: true:false
false :: true:false
fib :: 0':s → 0':s
fibiter :: 0':s → 0':s → 0':s → 0':s → 0':s
if :: true:false → 0':s → 0':s → 0':s → 0':s → 0':s
hole_0':s1_0 :: 0':s
hole_true:false2_0 :: true:false
gen_0':s3_0 :: Nat → 0':s

Lemmas:
plus(gen_0':s3_0(n5_0), gen_0':s3_0(b)) → gen_0':s3_0(+(n5_0, b)), rt ∈ Ω(1 + n5₀)

Generator Equations:
gen_0':s3_0(0) ⇔ 0'
gen_0':s3_0(+(x, 1)) ⇔ s(gen_0':s3_0(x))

No more defined symbols left to analyse.

(21) LowerBoundsProof (EQUIVALENT transformation)

The lowerbound Ω(n¹) was proven with the following lemma:
plus(gen_0':s3_0(n5_0), gen_0':s3_0(b)) → gen_0':s3_0(+(n5_0, b)), rt ∈ Ω(1 + n5₀)

(22) BOUNDS(n^1, INF)