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A204520 Numbers such that floor(a(n)^2 / 5) is a square. 20
0, 1, 2, 3, 7, 9, 18, 47, 123, 161, 322, 843, 2207, 2889, 5778, 15127, 39603, 51841, 103682, 271443, 710647, 930249, 1860498, 4870847, 12752043, 16692641, 33385282 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,3

COMMENTS

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.)

LINKS

Table of n, a(n) for n=1..27.

FORMULA

A204520(n) = sqrt(A055812(n)).

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

PROG

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

CROSSREFS

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

Sequence in context: A140189 A165803 A291740 * A007649 A218909 A225202

Adjacent sequences:  A204517 A204518 A204519 * A204521 A204522 A204523

KEYWORD

nonn

AUTHOR

M. F. Hasler, Jan 15 2012

STATUS

approved

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Last modified January 19 20:01 EST 2019. Contains 319309 sequences. (Running on oeis4.)