1,4
Eric Weisstein's World of Mathematics, Link to a section of The World of Mathematics.
Cf. A047713.
Sequence in context: A172212 A060621 A058880 * A073403 A064518 A135178
Adjacent sequences: A055548 A055549 A055550 * A055552 A055553 A055554
nonn,more
Eric Weisstein (eric(AT)weisstein.com)