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%I #18 Dec 31 2020 11:11:15
%S 6,9,15,20,24,31,35,42,49,59,63,72,76,84,95,106,110,121,125
%N a(n) is the number of faces of the stepped pyramid with n levels described in A245092.
%C To calculate a(n) consider that levels greater than n do not exist.
%C The shape of the n-th level of the pyramid allows us to know if n is prime (see the Formula section).
%C For more information about the sequences that we can see in the pyramid see A262626.
%H Omar E. Pol, <a href="http://www.polprimos.com/imagenespub/polpyr05.jpg">Perspective view of the pyramid (first 16 levels)</a>
%F a(n) = A325301(n) - A325302(n) + 2 (Euler's formula).
%F a(n) = A323645(n) + 3.
%F a(n) = a(n-1) + 4 iff n is a prime > 3 (A215848).
%e For n = 1 the first level of the stepped pyramid (starting from the top) is a cube, and a cube has six faces, so a(1) = 6.
%Y Cf. A325301 (number of edges), A325302 (number of vertices).
%Y Cf. A196020, A215848, A235791, A236104, A237270, A237271, A237591, A237593, A245092, A262626, A299692, A323645, A323648.
%K nonn,more
%O 1,1
%A _Omar E. Pol_, Apr 16 2019
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