This site is supported by donations to The OEIS Foundation.

Annual Appeal: Please make a donation (tax deductible in USA) to keep the OEIS running. Over 4500 articles have referenced us, often saying "we would not have discovered this result without the OEIS".

 Hints (Greetings from The On-Line Encyclopedia of Integer Sequences!)
 A129831 Alternating sum of double factorials: n!! - (n-1)!! + (n-2)!! - ... 1!!. 2
 1, 1, 2, 6, 9, 39, 66, 318, 627, 3213, 7182, 38898, 96237, 548883, 1478142, 8843778, 25615647, 160178913, 494550162, 3221341038, 10527969537, 71221636863, 245012506362, 1716978047238, 6188875533387, 44822878860213 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,3 LINKS FORMULA a(n) = n!! - (n-1)!! + (n-2)!! - ... 1!! = A006882(n) - a(n-1). EXAMPLE a(5) = 5!! - 4!! + 3!! - 2!! + 1!! = 15 - 8 + 3 - 2 + 1 = 9. MAPLE A129831 := proc(n)         add( (-1)^i*doublefactorial(n-i), i=0..n-1) ; end proc: # R. J. Mathar, Aug 22 2012 CROSSREFS Cf. A005165, A006882. Sequence in context: A242421 A093397 A082459 * A101713 A240761 A117541 Adjacent sequences:  A129828 A129829 A129830 * A129832 A129833 A129834 KEYWORD nonn,changed AUTHOR Paolo P. Lava & Giorgio Balzarotti, May 21 2007 STATUS approved

Lookup | Welcome | Wiki | Register | Music | Plot 2 | Demos | Index | Browse | More | WebCam
Contribute new seq. or comment | Format | Transforms | Superseeker | Recent | More pages
The OEIS Community | Maintained by The OEIS Foundation Inc.

Content is available under The OEIS End-User License Agreement .