A256481 Smallest prime obtained by appending a number with identical digits to n or 0 if no such prime exists.
2, 11, 23, 31, 41, 53, 61, 71, 83, 97, 101, 113, 127, 131, 149, 151, 163, 173, 181, 191, 2011, 211, 223, 233, 241, 251, 263, 271, 281, 293, 307, 311, 3299, 331, 347, 353, 367, 373, 383, 397, 401, 419, 421, 431, 443, 457, 461, 479, 487, 491, 503, 511111, 521, 5333
Offset: 0
Links
- Chai Wah Wu, Table of n, a(n) for n = 0..6068
- Chai Wah Wu, On a conjecture regarding primality of numbers constructed from prepending and appending identical digits, arXiv:1503.08883 [math.NT], 2015.
Programs
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Python
from gmpy2 import mpz, digits, is_prime def A256481(n,limit=2000): if n in (6930,50358,56574,72975): return 0 if n == 0: return 2 sn = str(n) for i in range(1,limit+1): for j in range(1,10,2): si = digits(j,10)*i p = mpz(sn+si) if is_prime(p): return int(p) else: return 'search limit reached.'
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