= Hisashi Ohtsuki
{c}
<Hisashi Ohtsuki> develops mathematical models of <cooperation>, connecting <population structure> with <natural selection>. His research asks how costly help can persist when individuals that withhold help can still receive its benefits.https://rcies.soken.ac.jp/scientists/scientists_hisashi_ohtsuki_en.html[Institutional research profile; mathematical biology and evolution of cooperation]{ref}
In a 2006 study, Ohtsuki and colleagues represented individuals by <graph vertices> and their interactions by <graph edges>. They calculated when a single cooperator had a <fixation probability> above its neutral benchmark. The result depended on how reproduction filled vacant sites, as well as on the <graph>. This helped make <network reciprocity> a quantitative mechanism: local interaction patterns and the updating rule can alter whether helping is favoured. The analysis used <weak selection> and large-population approximations, so the resulting threshold must be read with those assumptions.<A simple rule for the evolution of cooperation on graphs and social networks by Hisashi Ohtsuki and others>[Nature 441, pp. 502–504; model, fixation criterion and update-rule comparison]{ref}
\Image[https://rcies.soken.ac.jp/img/scientists/img_scientists06.png]
{title=Hisashi Ohtsuki}
{description=Portrait identified on the https://rcies.soken.ac.jp/scientists/scientists_hisashi_ohtsuki_en.html[SOKENDAI institutional profile].}
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