A285985 - OEIS
A285985
Numbers a(n) = (T(b(n)))^2, where T(b(n)) is the triangular number of b(n)= A000217(b(n)) and b(n)=A006451(n). Also a(n) = parameters K of the Bachet Mordell equation y^2=x^3+K, where x= T(b(n)) = A006454(n) and y= T(b(n))* sqrt(T(b(n))+1) = A285955(n)
3
0, 9, 225, 14400, 278784, 16769025, 322382025, 19356600384, 372051201600, 22337675375625, 429347532814209, 25777663981977600, 495466706924481600, 29747402099825117409, 571768151330225342025, 34328476252406392070400, 659819951198501829398784, 39615031848108328736769225, 761431651915943270106720225, 45715712424248689455481003584, 878691466491082103705616000000
COMMENTS
Numbers a(n) which are the square of triangular number T(b(n)), where b(n) is the sequence A006451(n) of numbers n such that T(n)+1 is a square.
This sequence a(n) gives also the parameters K of the 3rd degree Diophantine Bachet-Mordell equation y^2=x^3+K, with x= T(b(n)) = A006454(n) and y= T(b(n))* sqrt(T(b(n))+1) = A285955(n).
REFERENCES
V. Pletser, On some solutions of the Bachet-Mordell equation for large parameter values, to be submitted, April 2017.
FORMULA
Since b(n) = 8*sqrt(T(b(n-2))+1)+ b(n-4) = 8*sqrt((b(n-2)*(b(n-2)+1)/2)+1)+ b(n-4), with b(-1)=-1, b(0)=0, b(1)=2, b(2)=5 (see A006451) and a(n) = T(b(n)) (this sequence), one has :
a(n) = ([8*sqrt((b(n-2)*(b(n-2)+1)/2)+1)+ b(n-4)]*[ 8*sqrt((b(n-2)*(b(n-2)+1)/2)+1)+ b(n-4)+1]/2)^2.
Empirical g.f.: 9*x*(1 + 24*x + 387*x^2 + 864*x^3 + 387*x^4 + 24*x^5 + x^6) / ((1 - x)*(1 - 34*x + x^2)*(1 - 6*x + x^2)*(1 + 6*x + x^2)*(1 + 34*x + x^2)). - Colin Barker, Apr 30 2017
EXAMPLE
For n=2, b(n)=5, a(n)=225.
For n=5, b(n)=90, a(n)= 16769025.
MAPLE
restart: bm2:=-1: bm1:=0: bp1:=2: bp2:=5: print ('0, 0', '1, 9', '2, 225'); for n from 3 to 1000 do b:= 8*sqrt((bp1^2+bp1)/2+1)+bm2; a:=(b*(b+1)/2)^2; print(n, a); bm2:=bm1; bm1:=bp1; bp1:=bp2; bp2:=b; end do: