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

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

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

a(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, A055812, A204502, A204514, A204516, A204518 and A004275, A001075, A001541 for the analog in bases 10,...,6 and 4, 3, 2.

Sequence in context: A378925 A327779 A291740 * A358392 A007649 A218909

Adjacent sequences: A204517 A204518 A204519 * A204521 A204522 A204523

Keyword

nonn

Author

M. F. Hasler, Jan 15 2012

Status

approved