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 A099179 Iterated tetrahedral numbers, starting at Tet(2) = 4. 6
 2, 4, 20, 1540, 609896980, 37811003218473324699257860, 9009555207802177724984164589516456320805205201729086740415363658290866918420 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS The next term, a(8), has 228 digits. - Harvey P. Dale, Dec 18 2012 REFERENCES J. H. Conway and R. K. Guy, The Book of Numbers, Copernicus Press, NY, 1996, p. 83. H. S. M. Coxeter, Polyhedral numbers, pp. 25-35 of R. S. Cohen, J. J. Stachel and M. W. Wartofsky, eds., For Dirk Struik: Scientific, historical and political essays in honor of Dirk J. Struik, Reidel, Dordrecht, 1974. J. V. Post, "Iterated Triangular Numbers", preprint. LINKS Eric Weisstein's World of Mathematics, "Tetrahedral Number." FORMULA Given the tetrahedral number formula Tet(n) = n*(n+1)*(n+2)/6, define a(1) = 2; a(2) = the 2nd tetrahedral number = 2*(2+1)*(2+2)/6 = 4; Define a(k+1) = Tet(a(k)) = a(k)*[a(k)+1]*[a(k)+2]/6. a(n)= A000292(a(n-1)). - R. J. Mathar, Jun 09 2008 EXAMPLE a(2) = Tet(2) = the 2nd tetrahedral number = 2*(2+1)*(2+2)/6 = 4; a(3) = Tet(Tet(2)) = the 4th tetrahedral number = 4*(4+1)*(4+2)/6 = 20; a(4) = Tet(Tet(Tet(2))) = the 20th tetrahedral number = 20*(20+1)*(20+2)/6 = 1540. MATHEMATICA NestList[(#(#+1)(#+2))/6&, 2, 6] (* Harvey P. Dale, Dec 18 2012 *) CROSSREFS Cf. A007501, A000292. Sequence in context: A325503 A087314 A326972 * A102049 A058522 A292534 Adjacent sequences:  A099176 A099177 A099178 * A099180 A099181 A099182 KEYWORD easy,nonn AUTHOR Jonathan Vos Post, Nov 15 2004 EXTENSIONS Corrected and extended by R. J. Mathar, Jun 09 2008 STATUS approved

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Last modified November 17 16:08 EST 2019. Contains 329241 sequences. (Running on oeis4.)