A theta graph of order ℓ, denoted by Θℓ,
is a cycle Cℓ with a chord, where ℓ≥4.
The following result was used implicitly in [3]
and is stated as a separate lemma
in [16, Lemma 2] and also used in [4] and [15].
Let F be a Θ-graph of order ℓ and ℓ>t≥2…