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A124371 a(n) is the number of earlier terms a(k) such that a(k)*n is a square. 2
0, 1, 1, 3, 1, 1, 1, 1, 7, 1, 1, 2, 1, 1, 1, 12, 1, 2, 1, 1, 1, 1, 1, 1, 19, 1, 3, 2, 1, 1, 1, 4, 1, 1, 1, 27, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 5, 38, 4, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 2, 51, 1, 1, 1, 1, 1, 1, 1, 5, 1, 1, 5, 2, 1, 1, 1, 4, 64, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 6, 1, 82, 1, 1 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,4

LINKS

Antti Karttunen, Table of n, a(n) for n = 1..65537

EXAMPLE

12*a(1) = 0 and 12*a(4) = 36 are the two squares made by multiplying earlier terms with 12. So a(12) = 2.

MATHEMATICA

f[l_List] := Append[l, Length[Select[Sqrt /@ ((Length[l] + 1)*l), IntegerQ]]]; Nest[f, {}, 105] (* Ray Chandler, Oct 29 2006 *)

PROG

(PARI)

up_to = 1001;

prepare_A124371(up_to) = { my(v = vector(up_to), c); v[1] = 0; for(n=2, up_to, c=0; for(k=1, n-1, c += issquare(v[k]*n)); v[n] = c); (v); };

v124371 = prepare_A124371(up_to);

A124371(n) = v124371[n]; \\ Antti Karttunen, Jul 21 2018

CROSSREFS

Cf. A124372.

Sequence in context: A220693 A208615 A058663 * A147989 A119329 A054724

Adjacent sequences:  A124368 A124369 A124370 * A124372 A124373 A124374

KEYWORD

easy,nonn

AUTHOR

Leroy Quet, Oct 27 2006

EXTENSIONS

Extended by Ray Chandler, Oct 29 2006

STATUS

approved

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Last modified February 22 18:24 EST 2020. Contains 332148 sequences. (Running on oeis4.)