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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
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.
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
Sequence in context: A258931 A204642 A204635 * A189812 A230830 A038645
KEYWORD
nonn
AUTHOR
Farideh Firoozbakht, Jan 22 2005
STATUS
approved