A182272 Starting with 1, smallest integer not yet in the sequence such that two neighboring digits in the sequence multiply to a composite.

1, 4, 2, 3, 5, 6, 7, 8, 9, 10, 14, 16, 18, 19, 20, 22, 23, 24, 25, 26, 27, 28, 29, 30, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 72, 73, 74, 75, 76

Offset

1,2

Comments

For this sequence, 0 is considered composite.

Links

Dominic McCarty, Table of n, a(n) for n = 1..10000

Prog

(Python)
a, z = [1], [1, 2, 3, 5, 7]
while len(a) < 100:
    k, s = 2, "2"
    while (k in a) or (a[-1] % 10 * int(s[0]) in z) or \
        any(int(s[n]) * int(s[n+1]) in z for n in range(0, len(s)-1)): s, k = str(k+1), k+1
    a.append(k)
print(a) # Dominic McCarty, Feb 05 2025

Crossrefs

Sequence in context: A247206 A228047 A188543 * A182273 A166016 A213196

Adjacent sequences: A182269 A182270 A182271 * A182273 A182274 A182275

Keyword

nonn,base

Author

Jim Nastos and Eric Angelini, Apr 22 2012

Status

approved