From the authors:
<blockquote>
... our choice of the odd integer closest to (2 ^^ 64)/φ was based
only on the intuition that it might be a good idea to keep γ values
“well spread out” and the fact that prefixes of the Weyl sequence
generated by 1/φ are known to be “well spread out” [citing vol. 3 of
Knuth's <i>The Art of Computer Programming</i>, exercise 6.4-9]
</blockquote>
Default increment used by SplitMixEngine. Defined in Fast Splittable Pseudorandom Number Generators (2014) as "the odd integer closest to (2 ^^ 64)/φ, where φ = (1 + √5)/2 is the golden ratio." In the paper this constant is referred to as "GOLDEN_GAMMA".
From the authors: <blockquote> ... our choice of the odd integer closest to (2 ^^ 64)/φ was based only on the intuition that it might be a good idea to keep γ values “well spread out” and the fact that prefixes of the Weyl sequence generated by 1/φ are known to be “well spread out” [citing vol. 3 of Knuth's <i>The Art of Computer Programming</i>, exercise 6.4-9] </blockquote>