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A000250 Number of symmetric reflexive relations on n nodes: (1/2)*A000666.
(Formerly M2868 N1153)
1
1, 3, 10, 45, 272, 2548, 39632, 1104306, 56871880, 5463113568, 978181717680, 326167542296048, 202701136710498400, 235284321080559981952, 511531711735594715527360, 2089424601541011618029114896, 16084004145036771186002041099712, 234026948449058790311618594954430848, 6454432593140577452393525511509194184320 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

REFERENCES

Harary, Frank; Palmer, Edgar M.; Robinson, Robert W.; Schwenk, Allen J.; Enumeration of graphs with signed points and lines. J. Graph Theory 1 (1977), no. 4, 295-308.

M. D. McIlroy, Calculation of numbers of structures of relations on finite sets, Massachusetts Institute of Technology, Research Laboratory of Electronics, Quarterly Progress Reports, No. 17, Sept. 15, 1955, pp. 14-22.

W. Oberschelp, Kombinatorische Anzahlbestimmungen in Relationen, Math. Ann., 174 (1967), 53-78.

N. J. A. Sloane, A Handbook of Integer Sequences, Academic Press, 1973 (includes this sequence).

N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence).

LINKS

Table of n, a(n) for n=1..19.

M. D. McIlroy, Calculation of numbers of structures of relations on finite sets, Massachusetts Institute of Technology, Research Laboratory of Electronics, Quarterly Progress Reports, No. 17, Sep. 15, 1955, pp. 14-22. [Annotated scanned copy]

CROSSREFS

Cf. A000595, A001173, A001174.

Sequence in context: A003704 A292181 A236409 * A118601 A005143 A270493

Adjacent sequences:  A000247 A000248 A000249 * A000251 A000252 A000253

KEYWORD

nonn,nice

AUTHOR

N. J. A. Sloane

EXTENSIONS

More terms from Vladeta Jovovic, Apr 18 2000

More terms from Herman Jamke (hermanjamke(AT)fastmail.fm), May 05 2007

STATUS

approved

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Last modified September 20 15:44 EDT 2017. Contains 292275 sequences.