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function(n)n%in%choose(3:n,3)
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Based on the observation on wiki or OEIS that \$Te_n={{n+2}\choose{3}}\$; : ranges downwards for the case of \$n=1\$.
:
function(n)n%in%choose(13:n+2n,3)
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function(n)n%in%choose(1:n+2,3)
function(n,`?`=cumsum)n%in%??n%in%choose(1:nn+2,3)
Double cumsum from the Lyxal answers.Try it online!
function(n,`?`=cumsum)n%in%??1:n
Double cumsum from the Lyxal answers.
