

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
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OFFSET

1,3


COMMENTS

Also: Numbers whose square, with its last base5 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  x1) / ((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



