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A102589 Numbers of the forms 4*p or q where both numbers p=(10^n-7)/3 and q=(127*10^(n-1)-7)/3 are primes. 1
124, 421, 1324, 4231, 13324, 42331, 1333324, 4233331 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

This sequence is a subsequence of A069514 (If m is in the sequence then sigma(reversal(m)) = reversal(sigma(m))), so see Comments on A069514. There exists only four known n's such that both numbers p=(10^n-7)/3 and q=(127*10^(n-1)-7)/3 are primes, n=2,3, 4 and 6.

LINKS

Table of n, a(n) for n=1..8.

EXAMPLE

124 and 421 are in the sequence because 124=4*31 and both numbers

31=(10^2-7)/3 and 421=(127*10^(2-1)-7)/3 are primes. It seems that

there is no further term.

CROSSREFS

Cf. A069514, A102285.

Sequence in context: A258931 A204642 A204635 * A189812 A230830 A038645

Adjacent sequences:  A102586 A102587 A102588 * A102590 A102591 A102592

KEYWORD

nonn

AUTHOR

Farideh Firoozbakht, Jan 22 2005

STATUS

approved

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Last modified September 20 21:48 EDT 2021. Contains 347591 sequences. (Running on oeis4.)