A338218 - OEIS

%I #11 Feb 14 2021 13:16:00

%S 1,1,1,1,2,3,6,12,23,43,80,140,233,387,612,930,1411,2067,2936,4170,

%T 5768,7796,10537,13960,18163,23639,30285,38249,48322,60285,74340,

%U 91706,111967,135403

%N Number of terms in polynomial sequence s(n) = x*y*z*(s(n-1)*s(n-3) + s(n-2)^2)/s(n-4), with s(1) = x, s(2) = s(3) = 1, s(4) = y.

%C The Somos-4 polynomial sequence is s(n).

%H Michael Somos, <a href="http://grail.eecs.csuohio.edu/~somos/somospol.html">Somos polynomials</a>

%F a(n) = a(5-n) for all n in Z.

%e a(6) = 3 because s(6) = x*y^3*z^2 + x*y^3*z + x*y^2*z^2 has 3 terms.

%t a[ n_] := If[1 <= n <= 4, 1, RecurrenceTable[{s[m]*s[m - 4] == x*y*z*(s[m - 1]*s[m - 3] + s[m - 2]^2), s[1] == x, s[2] == 1, s[3] == 1, s[4] == y}, s, {m, Max[n, 5 - n]}] // Last // Factor // Expand // Length];

%Y Cf. A333260.

%K nonn,more

%O 1,5

%A _Michael Somos_, Jan 29 2021

%E a(31)-a(34) from _Jinyuan Wang_, Feb 14 2021