login
This site is supported by donations to The OEIS Foundation.

 

Logo


Hints
(Greetings from The On-Line Encyclopedia of Integer Sequences!)
A081975 Triangular number pertaining to A081974. a(n) = A081974(n)*A081974(n+1). 3
3, 6, 10, 45, 36, 28, 91, 78, 66, 231, 210, 120, 276, 1035, 630, 378, 1431, 1378, 780, 990, 528, 496, 465, 300, 820, 3321, 3240, 2080, 5356, 5253, 1275, 1225, 1176, 1128, 4371, 4278, 4186, 5460, 4560, 11476, 11325, 2775, 666, 2016, 12880, 6555, 1596 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

A subset of A000217. - R. J. Mathar, Apr 05 2007

LINKS

Table of n, a(n) for n=1..47.

MAPLE

isA000217 := proc(n) local t; t := (sqrt(1+8*n)-1)/2 ; type(t, 'integer'); end: A081974 := proc(nmax) local a, n, prodset; a := [1, 3] ; prodset := {3} ; while nops(a) < nmax do n := 2 ; while n in a or n*op(-1, a) in prodset or isA000217(n*op(-1, a)) = false do n := n+1 ; od ; prodset := prodset union { n*op(-1, a) } ; a := [op(a), n] ; od ; RETURN(a) ; end: A081975 := proc(nmax) local a ; a081974 := A081974(nmax) ; a := [] ; for i from 2 to nops(a081974) do a := [op(a), op(i, a081974)*op(i-1, a081974)] ; od ; RETURN(a) ; end: a := A081975(100) ; # R. J. Mathar, Apr 05 2007

CROSSREFS

Cf. A081974, A081976, A081977.

Sequence in context: A262242 A317361 A048089 * A160965 A069708 A061455

Adjacent sequences:  A081972 A081973 A081974 * A081976 A081977 A081978

KEYWORD

nonn

AUTHOR

Amarnath Murthy, Apr 03 2003

EXTENSIONS

More terms from R. J. Mathar, Apr 05 2007

STATUS

approved

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

License Agreements, Terms of Use, Privacy Policy. .

Last modified November 19 22:34 EST 2019. Contains 329323 sequences. (Running on oeis4.)