%I
%S 1,2,1,3,3,1,4,6,4,1,5,10,10,5,1,6,15,20,15,6,1,7,21,35,35,21,7,1,8,
%T 28,56,70,56,28,8,1,9,36,84,126,126,84,36,9,1,10,45,120,210,252,210,
%U 120,45,10,1,11,55,165,330,462,462,330,165,55,11,1,12,66,220,495,792,924,792
%N Triangle read by rows, giving the numbers T(n,m) = binomial(n+1,m+1); or, Pascal's triangle A007318 with its lefthand edge removed.
%C T(n,m) is the number of mfaces of a regular nsimplex.
%C An nsimplex is the ndimensional analogue of a triangle. Specifically, a simplex is the convex hull of a set of (n + 1) affinely independent points in some Euclidean space of dimension n or higher, i.e., a set of points such that no mplane contains more than (m + 1) of them. Such points are said to be in general position.
%C Reversing the rows gives A074909, which as a linear sequence is essentially the same as this.
%C From _Tom Copeland_, Dec 07 2007: (Start)
%C T(n,k) * (k+1)! = A068424. The comment on permuted words in A068424 shows that T is related to combinations of letters defined by connectivity of regular polytope simplexes.
%C If T is the diagonallyshifted Pascal matrix, binomial(n+m,k+m), for m=1, then T is a fundamental type of matrix that is discussed in A133314 and the following hold.
%C The infinitesimal matrix generator is given by A132681, so T = LM(1) of A132681 with inverse LM(1).
%C With a(k) = (x)^k / k!, T * a = [ Laguerre(n,x,1) ], a vector array with index n for the Laguerre polynomials of order 1. Other formulae for the action of T are given in A132681.
%C T(n,k) = (1/n!) (D_x)^n (D_t)^k Gf(x,t) evaluated at x=t=0 with Gf(x,t) = exp[ t * x/(1x) ] / (1x)^2.
%C [O.g.f. for T ] = 1 / { [ 1 + t * x/(1x) ] * (1x)^2 }. [ O.g.f. for row sums ] = 1 / { (1x) * (12x) }, giving A000225 (without a leading zero) for the row sums. Alternating sign row sums are all 1.
%C O.g.f. for row polynomials = [ (1+q)**(n+1)  1 ] / [ (1+q) 1 ] = A(1,n+1,q) on page 15 of reference on Grassmann cells in A008292. (End)
%C Given matrices A and B with A(n,k) = T(n,k)*a(nk) and B(n,k) = T(n,k)*b(nk), then A*B = C where C(n,k) = T(n,k)*[a(.)+b(.)]^(nk), umbrally. The e.g.f. for the row polynomials of A is {(a+t) exp[(a+t)x]  a exp(a x)}/t, umbrally.  _Tom Copeland_, Aug 21 2008
%C A007318*A097806 as infinite lower triangular matrices.  _Philippe Deléham_, Feb 08 2009
%C Riordan array (1/(1x)^2, x/(1x)).  _Philippe Deléham_, Feb 22 2012
%C The elements of the matrix inverse are T^(1)(n,k)=(1)^(n+k)*T(n,k).  _R. J. Mathar_, Mar 12 2013
%C Relation to Ktheory: T acting on the column vector (0,d,d^2,d^3,...) generates the Euler classes for a hypersurface of degree d in CP^n. Cf. Dugger p. 168 and also A104712, A111492, and A238363.  _Tom Copeland_, Apr 11 2014
%C Number of walks of length p>0 between any two distinct vertices of the complete graph K_(n+2) is W(n+2,p)=(1)^(p1)*sum(k=0,..,p1, T(p1,k)*(n2)^k) = [(n+1)^p(1)^p]/(n+2) = (1)^(p1)*sum(k=0,..,p1, (n1)^k). This is equal to (1)^(p1)*Phi(p,n1), where Phi is the cyclotomic polynomial when p is an odd prime. For K_3, see A001045; for K_4, A015518; for K_5, A015521; for K_6, A015531; for K_7, A015540.  _Tom Copeland_, Apr 14 2014
%H Tom Copeland, <a href="http://mathoverflow.net/questions/82560/cyclotomicpolynomialsincombinatorics">Cyclotomic polynomials in combinatorics</a>
%H D. Dugger, <a href="http://math.uoregon.edu/~ddugger/kgeom.pdf">A Geometric Introduction to KTheory</a> [From _Tom Copeland_, Apr 11 2014]
%H B. Grünbaum and G. C. Shephard, <a href="http://dx.doi.org/10.1112/blms/1.3.257">Convex polytopes</a>, Bull. London Math. Soc. (1969) 1 (3): 257300.
%H Justin Hughes, <a href="http://www.groupsstandrews.org/2013/slides/Hughes.pdf ">Representations Arising from an Action on Dneighborhoods of Cayley Graphs</a>, 2013; slides from a talk.
%H Wikipedia, <a href="http://en.wikipedia.org/wiki/Simplex">Simplex</a>
%F T(n,m) = sum(binomial(k,m),k=m..n) = binomial(n+1,m+1), n>=m>=0, else 0. (partial sum of column m of A007318 (Pascal), or summation on the upper binomial index (Graham et al. (GKP), eq.(5.10)). For the GKP reference see A007318).  _Wolfdieter Lang_, Aug 22 2012
%F E.g.f.: 1/x*((1 + x)*exp(t*(1 + x))  exp(t)) = 1 + (2 + x)*t + (3 + 3*x + x^2)*t^2/2! + .... The infinitesimal generator for this triangle has the sequence [2,3,4,...] on the main subdiagonal and 0's elsewhere.  _Peter Bala_, Jul 16 2013
%F T(n,k)=2*T(n1,k)+T(n1,k1)T(n2,k)T(n2,k1), T(0,0)=1, T(1,0)=2, T(1,1)=1, T(n,k)=0 if k<0 or if k>n.  _Philippe Deléham_, Dec 27 2013
%F T(n,k) = A193862(n,k)/2^k.  _Philippe Deléham_, Jan 29 2014
%F G.f.: 1/((1x)*(1xx*y)).  _Philippe Deléham_, Mar 13 2014
%F From Copeland's 2007 and 2008 comments:
%F A) O.g.f.: 1 / { [ 1 + t * x/(1x) ] * (1x)^2 } (same as Deleham's).
%F B) The infinitesimal generator for T is given in A132681 with m=1 (same as Bala's), which makes connections to the ubiquitous associated Laguerre polynomials of integer orders, for this case the Laguerre polynomials of order one L(n,t,1).
%F C) O.g.f. of row e.g.f.s: sum(n=0,1,..infinity, L(n,t,1) x^n) = exp[t*x/(1x)]/(1x)^2 = 1 + (2+t)x + (3+3*t+t^2/2!)x^2 + (4+6*t+4*t^2/2!+t^3/3!)x^3+ ... .
%F D) E.g.f. of row o.g.f.s: ((1+t)*exp((1+t)*x)exp(x))/t (same as Bala's).
%F E) E.g.f. for T(n,k)*a(nk): {(a+t) exp[(a+t)x]  a exp(a x)}/t, umbrally. For example, for a(k)=2^k, the e.g.f. for the row o.g.f.s is {(2+t) exp[(2+t)x]  2 exp(2x)}/t.
%F (End)  _Tom Copeland_, Mar 26 2014
%e Triangle begins:
%e 1
%e 2, 1
%e 3, 3, 1
%e 4, 6, 4, 1
%e 5, 10, 10, 5, 1
%e 6, 15, 20, 15, 6, 1
%e Production matrix begins
%e 2...1
%e 1..1...1
%e 1...0...1...1
%e 1..0...0...1...1
%e 1...0...0...0...1...1
%e 1..0...0...0...0...1...1
%e 1...0...0...0...0...0...1...1
%e 1..0...0...0...0...0...0...1...1
%e 1...0...0...0...0...0...0...0...1...1
%e  _Philippe Deléham_, Jan 29 2014
%p for i from 0 to 12 do seq(binomial(i, j)*1^(ij), j = 1 .. i) od;
%Y Cf. A007318, A014410, A228196.
%K easy,nonn,tabl,changed
%O 0,2
%A _Zerinvary Lajos_, Dec 02 2007
%E Edited by _Tom Copeland_ and _N. J. A. Sloane_, Dec 11 2007
%E Typo corrected in my 2008 comment by _Tom Copeland_, Mar 28 2014
