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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 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 March 26 17:05 EDT 2023. Contains 361551 sequences. (Running on oeis4.)