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  \code{nextn()} is intended to be used to find a suitable length
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  \code{nextn()} is intended to be used to find a suitable length
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  to zero-pad the argument of \code{\link{fft}}
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  to zero-pad the argument of \code{\link{fft}}
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  so that the transform is computed quickly.
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  so that the transform is computed quickly.
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  The default value for \code{factors} ensures this.
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  The default value for \code{factors} ensures this.
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}
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}
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\value{a vector of the same \code{\link{length}} as \code{n}, of type
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  \code{"integer"} when the values are small enough (determined before
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  computing them) and \code{"double"} otherwise.
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}
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\note{If the factors in \code{factors} are \emph{not} relative prime,
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\note{If the factors in \code{factors} are \emph{not} relative prime,
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  i.e., have themselves a common factor larger than one, the result may
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  i.e., have themselves a common factor larger than one, the result may
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  be wrong in the sense that it may not be the \emph{smallest} integer.
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  be wrong in the sense that it may not be the \emph{smallest} integer.
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  E.g., \code{nextn(91, c(2,6))} returns 128 instead of 96 as
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  E.g., \code{nextn(91, c(2,6))} returns 128 instead of 96 as
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  \code{nextn(91, c(2,3))} returns.
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  \code{nextn(91, c(2,3))} returns.
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  When the resulting \code{N <- nextn(..)} is larger than \code{2^53}, a
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  warning with the true 64-bit integer value is signalled, as integers
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  above that range may not be representable in double precision.
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  If you really need to deal with such large integers, it may be
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  advisable to use package \CRANpkg{gmp}.
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}
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}
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\seealso{
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\seealso{
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  \code{\link{convolve}}, \code{\link{fft}}.
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  \code{\link{convolve}}, \code{\link{fft}}.
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}
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}
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\examples{
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\examples{