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.

124, 421, 1324, 4231, 13324, 42331, 1333324, 4233331

Offset: 1

Author

Farideh Firoozbakht, Jan 22 2005

Keywords

• nonn

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.

Examples

			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.