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A204520 Numbers such that floor(a(n)^2 / 5) is a square. 19

%I

%S 0,1,2,3,7,9,18,47,123,161,322,843,2207,2889,5778,15127,39603,51841,

%T 103682,271443,710647,930249,1860498,4870847,12752043,16692641,

%U 33385282

%N Numbers such that floor(a(n)^2 / 5) is a square.

%C Also: Numbers whose square, with its last base-5 digit dropped, is again a square. (For the three initial terms whose squares have only one digit in base 5, it is then understood that this yields zero.)

%F a(n) = sqrt(A055812(n)).

%F Empirical g.f.: -x^2*(x+1)*(3*x^6 + 4*x^5 + 14*x^4 - 5*x^3 - 2*x^2 - x-1) / ((x^4 - 4*x^2 - 1)*(x^4 + 4*x^2 - 1)). - _Colin Barker_, Sep 15 2014

%o (PARI) b=5;for(n=0,2e9,issquare(n^2\b) && print1(n","))

%Y Cf. A031149, A055812, A204502, A204514, A204516, A204518 and A004275, A001075, A001541 for the analog in bases 10,...,6 and 4, 3, 2.

%K nonn

%O 1,3

%A _M. F. Hasler_, Jan 15 2012

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Last modified June 13 00:57 EDT 2021. Contains 344980 sequences. (Running on oeis4.)