login
A178901
a(1)=2; for n > 1, a(n) is the largest number <= 2*a(n-1) divisible by n.
2
2, 4, 6, 12, 20, 36, 70, 136, 270, 540, 1078, 2148, 4290, 8568, 17130, 34256, 68510, 137016, 274018, 548020, 1096032, 2192058, 4384099, 8768184, 17536350, 35072700, 70145379, 140290752, 280581496, 561162990, 1122325953, 2244651904
OFFSET
1,1
COMMENTS
The definition "a(1)=1; for n > 1, a(n) is the largest number <= 2*a(n-1) divisible by n" produces the natural numbers 1,2,3,4,5,... (A000027).
EXAMPLE
2*36=72. But 72 is not a multiple of 7, so we must look for the largest multiple of 7 <= 72, and this is 70.
MAPLE
A178901 := proc(n) option remember; if n =1 then 2; else for a from 2*procname(n-1) by -1 do if a mod n = 0 then return a; end if; end do: end if; end proc: seq(A178901(n), n=1..30) ; # R. J. Mathar, Jun 26 2010
PROG
(PARI) lista(nn) = {my(va = vector(nn)); va[1] = 2; for (n=2, nn, va[n] = n*(2*va[n-1]\n); ); va; } \\ Michel Marcus, Dec 18 2020
CROSSREFS
Cf. A000027.
Sequence in context: A294429 A107383 A078025 * A164146 A370582 A279245
KEYWORD
nonn
AUTHOR
J. Lowell, Jun 21 2010
EXTENSIONS
a(8)-a(32) from Robert G. Wilson v and John W. Layman, Jun 28 2010
STATUS
approved