A102589 - OEIS

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 (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=431 and both numbers` `31=(10^2-7)/3 and 421=(12710^(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`

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