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A264815
Semirps: a semirp (or semi-r-p) is a semiprime r*p with r and p both reversed primes.
1
4, 6, 9, 10, 14, 15, 21, 22, 25, 26, 33, 34, 35, 39, 49, 51, 55, 62, 65, 74, 77, 85, 91, 93, 111, 119, 121, 142, 143, 146, 155, 158, 169, 185, 187, 194, 202, 213, 214, 217, 219, 221, 226, 237, 259, 262, 289, 291, 298, 302, 303, 314, 321, 334, 339, 341, 355
OFFSET
1,1
COMMENTS
A semiprime (A001358) is the product of two prime, not necessarily distinct. A semiprime is in this list if those two primes (A000040) are reversed primes (A004087).
Since A007500 is the intersection of A000040 and A004087, this sequence is also the sorted list of all r*p with r and p in A007500.
LINKS
FORMULA
[A007500]^2, sorted.
EXAMPLE
9 is in the list because 9 = 3*3 is a semiprime and reverse(3) = 3 is prime.
143 is in the list because 143 = 11*13 is a semiprime and both reverse(11) = 11 and reverse(13) = 31 are prime.
PROG
(Sage)
reverse = lambda n: sum([10^i*int(str(n)[i]) for i in range(len(str(n)))])
def is_semirp(n):
F = factor(n)
if sum([f[1] for f in F])==2:
r, p = F[0][0], F[-1][0]
if is_prime(reverse(r)) and is_prime(reverse(p)): return True
[a for a in range(1, 356) if is_semirp(a)] # Danny Rorabaugh, Nov 25 2015
CROSSREFS
KEYWORD
nonn,base
AUTHOR
Danny Rorabaugh, Nov 25 2015
STATUS
approved