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A279043
Numbers k such that 3*k+1 and 4*k+1 are both triangular numbers (A000217).
1
63, 12285, 2383290, 462346038, 89692748145, 17399930794155, 3375496881317988, 654828995044895580, 127033449541828424595, 24643834382119669475913, 4780776836681674049902590, 927446062481862646011626610, 179919755344644671652205659813
OFFSET
1,1
FORMULA
a(n) = 195*a(n-1) - 195*a(n-2) + a(n-3) for n>3.
G.f.: 63*x / ((1 - x)*(1 - 194*x + x^2)).
EXAMPLE
63 is in the sequence because 3*63+1 = 190 and 4*63+1 = 253 are both triangular numbers.
PROG
(PARI) Vec(63*x / ((1 - x)*(1 - 194*x + x^2)) + O(x^20))
(PARI) isok(k) = ispolygonal(3*k+1, 3) & ispolygonal(4*k+1, 3)
CROSSREFS
Sequence in context: A174763 A160871 A183525 * A234629 A270507 A289875
KEYWORD
nonn,easy
AUTHOR
Colin Barker, Dec 04 2016
STATUS
approved