A350831 a(n) is the least positive integer not listed earlier such that its first digit together with a(n-1)'s last digit can form a prime; a(1) = 1.

1, 3, 2, 9, 5, 31, 4, 7, 6, 11, 12, 32, 33, 8, 34, 13, 14, 15, 35, 36, 16, 17, 18, 37, 19, 21, 38, 39, 22, 91, 41, 42, 92, 93, 23, 24, 71, 43, 25, 94, 72, 95, 96, 73, 26, 74, 75, 97, 44, 76, 77, 45, 98, 99, 27, 46, 78, 301, 47, 48, 302, 303, 28, 304, 79, 29

Offset

1,2

Comments

Conjectured to contain all positive integers not multiple of 10.

Links

Table of n, a(n) for n=1..66.

Éric Angelini, Prime bayonets, personal web site "Cinquante signes" on blogspot.com, May 03 2022.

Prog

(PARI) u=[a=1]; vector(99, n, if(a>u[1]+1, u=setunion(u, [a]), u[1]=a; while(#u>1 && u[2]==u[1]+1+(u[1]%10==9), u=u[^1])); a%=10; my(k=u[1], d); n>1&& until(k++%10 && !setsearch(u, k) && (isprime(10*a+d=k\10^logint(k, 10)) || isprime(10*d+a)), ); a=k)

Crossrefs

Cf. A350832 for a more complicated 2-digit variant.

Sequence in context: A231442 A319107 A228323 * A140590 A329211 A164279

Adjacent sequences: A350828 A350829 A350830 * A350832 A350833 A350834

Keyword

nonn,base

Author

M. F. Hasler and Eric Angelini, May 03 2022

Status

approved