

A016742


Even squares: a(n) = (2*n)^2.


144



0, 4, 16, 36, 64, 100, 144, 196, 256, 324, 400, 484, 576, 676, 784, 900, 1024, 1156, 1296, 1444, 1600, 1764, 1936, 2116, 2304, 2500, 2704, 2916, 3136, 3364, 3600, 3844, 4096, 4356, 4624, 4900, 5184, 5476, 5776, 6084, 6400, 6724, 7056, 7396, 7744, 8100, 8464
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OFFSET

0,2


COMMENTS

4 times the squares.
It is conjectured (I think) that a regular Hadamard matrix of order n exists iff n is an even square (cf. Seberry and Yamada, Th. 10.11). A Hadamard matrix is regular if the sum of the entries in each row is the same.  N. J. A. Sloane, Nov 13 2008
Sequence arises from reading the line from 0, in the direction 0, 16, ... and the line from 4, in the direction 4, 36, ... in the square spiral whose vertices are the squares A000290.  Omar E. Pol, May 24 2008
The entries from a(1) on can be interpreted as pair sums of (2, 2), (8, 8), (18, 18), (32, 32) etc. that arise from a rearrangement of the subshell orbitals in the periodic table of elements. 8 becomes the maximum number of electrons in the (2s,2p) or (3s,3p) orbitals, 18 the maximum number of electrons in (4s,3d,4p) or (5s,3d,5p) shells, for example.  Julio Antonio Gutiérrez Samanez, Jul 20 2008
The first two terms of the sequence (n=1, 2) give the numbers of chemical elements using only n types of atomic orbitals, i.e., there are a(1)=4 elements (H,He,Li,Be) where electrons reside only on sorbitals, there are a(2)=16 elements (B,C,N,O,F,Ne,Na,Mg,Al,Si,P,S,Cl,Ar,K,Ca) where electrons reside only on s and porbitals. However, after that, there is 37 (which is one more than a(3)=36) elements (from Sc, Scandium, atomic number 21 to La, Lanthanum, atomic number 57) where electrons reside only on s, p and dorbitals. This is because Lanthanum (with the electron configuration [Xe]5d^1 6s^2) is an exception to the Aufbau principle, which would predict that its electron configuration is [Xe]4f^1 6s^2.  Antti Karttunen, Aug 14 2008.
Number of cycles of length 3 in the king's graph associated with an (n+1) X (n+1) chessboard.  Anton Voropaev (anton.n.voropaev(AT)gmail.com), Feb 01 2009
The sum to infinity of the reciprocals of the members of this sequence converges to (1/4)*Pi^2/6 = Pi^2/24.  Ant King, Nov 04 2009
a(n+1) is the molecular topological index of the nstar graph S_n.  Eric W. Weisstein, Jul 11 2011
a(n) is the sum of two consecutives odd numbers 2*n^21 and 2*n^2+1 and the difference of two squares (n^2+1)^2  (n^21)^2.  Pierre CAMI, Jan 02 2012
For n > 3, a(n) is the area of the irregular quadrilateral created by the points ((n4)*(n3)/2,(n3)*(n2)/2), ((n2)*(n1)/2,(n1)*n/2), ((n+1)*(n+2)/2,n*(n+1)/2), and ((n+3)*(n+4)/2,(n+2)*(n+3)/2).  J. M. Bergot, May 27 2014
a(n) is the least number k = x + y, with x>0 and y>0, such that there are n different pairs (x,y) for which x*y/k is an integer.  Paolo P. Lava, Jan 26 2018
Number of terms less than 10^k: 1, 2, 5, 16, 50, 159, 500, 1582, 5000, 15812, 50000, 158114, 500000, ...  Muniru A Asiru, Jan 28 2018
Righthand side of the binomial coefficient identity Sum_{k = 0..2*n} (1)^(k+1)* binomial(2*n,k)*binomial(2*n + k,k)*(2*n  k) = a(n).  Peter Bala, Jan 12 2022


REFERENCES

R. L. Graham, D. E. Knuth and O. Patashnik, Concrete Mathematics. AddisonWesley, Reading, MA, 2nd ed., 1994, p. 99.
Seberry, Jennifer and Yamada, Mieko; Hadamard matrices, sequences and block designs, in Dinitz and Stinson, eds., Contemporary design theory, pp. 431560, WileyIntersci. Ser. Discrete Math. Optim., Wiley, New York, 1992.
W. D. Wallis, Anne Penfold Street and Jennifer Seberry Wallis, Combinatorics: Room squares, sumfree sets, Hadamard matrices, Lecture Notes in Mathematics, Vol. 292, SpringerVerlag, BerlinNew York, 1972. iv+508 pp.


LINKS

Eric Weisstein's World of Mathematics, King Graph.


FORMULA

a(n) = 3*a(n1)  3*a(n2) + a(n3) with a(0) = 0, a(1) = 4, a(2) = 16.  Philippe Deléham, Mar 26 2013
Product_{n>=1} (1 + 1/a(n)) = sinh(Pi/2)/(Pi/2) (A308716).
Product_{n>=1} (1  1/a(n)) = sin(Pi/2)/(Pi/2) = 2/Pi (A060294). (End)


MAPLE



MATHEMATICA



PROG

(Maxima) makelist((2*n)^2, n, 0, 20); /* Martin Ettl, Jan 22 2013 */
(Haskell)
a016742 = (* 4) . (^ 2)
a016742_list = 0 : map (subtract 4) (zipWith (+) a016742_list [8, 16 ..])


CROSSREFS

Cf. A000290, A001105, A001539, A016754, A016802, A016814, A016826, A016838, A007742, A033991, A245058.


KEYWORD

nonn,easy


AUTHOR



EXTENSIONS

More terms from Sabir AbdusSamee (sabdulsamee(AT)prepaidlegal.com), Mar 13 2006


STATUS

approved



