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A246805 Lexicographically earliest sequence of distinct terms such that, when i<j, at least one of a(i) U a(j) or a(j) U a(i) is prime (where U denotes concatenation). 1
1, 3, 4, 7, 19, 31, 67, 391, 583, 4549, 917467, 6777061, 86794921, 1421517037 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

COMMENTS

Two distinct terms can always be concatenated in some way to form a prime number.

Is this sequence infinite?

LINKS

Table of n, a(n) for n=1..14.

Paul Tek, PARI program for this sequence

EXAMPLE

The following concatenations are prime:

- j=2: a(1) U a(2)=13, a(2) U a(1)=31

- j=3: a(3) U a(1)=41, a(3) U a(2)=43

- j=4: a(1) U a(4)=17, a(4) U a(1)=71, a(2) U a(4)=37, a(4) U a(2)=73, a(3) U a(4)=47

- j=5: a(5) U a(1)=191, a(5) U a(2)=193, a(3) U a(5)=419, a(4) U a(5)=719, a(5) U a(4)=197

- j=6: a(1) U a(6)=131, a(6) U a(1)=311, a(2) U a(6)=331, a(6) U a(2)=313, a(3) U a(6)=431, a(6) U a(4)=317, a(5) U a(6)=1931, a(6) U a(5)=3119

PROG

(PARI) See Link section.

CROSSREFS

Cf. A156770, A228323.

Sequence in context: A042227 A117789 A169892 * A241660 A030724 A124082

Adjacent sequences:  A246802 A246803 A246804 * A246806 A246807 A246808

KEYWORD

base,nonn,more

AUTHOR

Paul Tek, Nov 16 2014

STATUS

approved

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Last modified February 20 14:58 EST 2020. Contains 332078 sequences. (Running on oeis4.)