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A219195 Least i > 1 such that the sum of the digits in the base-j representation of i is invariant over all j in the first n squarefree numbers. 0
2, 6, 6, 687, 1386, 82305020 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

LINKS

Table of n, a(n) for n=1..6.

EXAMPLE

For n = 4 the number 687 is the least i > 1 such that s_2 (i) = s_3 (i) = s_5 (i) = s_6 (i), where s_k (i) is the sum of the digits in the base-k representation of i.

PROG

(PARI) tsqf(n) = {sqfs = vector(n); ipos = 1; na = 2; while (ipos <= n, if (issquarefree(na), sqfs[ipos] = na; ipos++; ); na++; ); return (sqfs); }

iok(i, sqfs) = {vdig = digits(i, sqfs[1]); sdig = sum(k=1, #vdig, vdig[k]); for (j=2, #sqfs, vdig = digits(i, sqfs[j]); if (sum(k=1, #vdig, vdig[k]) != sdig, return(0)); ); return (1); }

a(n) = {sqfs = tsqf(n); ok = 0; i = 2; while(! ok, if (iok(i, sqfs), ok = 1, i++); ); return (i); } \\ Michel Marcus, May 25 2013

CROSSREFS

Cf. A005117 (squarefree numbers).

Sequence in context: A189144 A130726 A279841 * A212223 A158915 A257478

Adjacent sequences:  A219192 A219193 A219194 * A219196 A219197 A219198

KEYWORD

nonn,base

AUTHOR

Jeffrey Shallit, Nov 14 2012

STATUS

approved

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Last modified June 26 13:14 EDT 2022. Contains 354883 sequences. (Running on oeis4.)