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A303137
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Numbers k such that concat(k, k-d(k)) and concat(k-d(k), k) are both prime, where d(k) is the number of divisors of k.
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1
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3, 29, 51, 53, 87, 177, 213, 291, 297, 357, 359, 399, 419, 427, 431, 471, 503, 521, 553, 561, 573, 597, 599, 659, 681, 687, 697, 719, 793, 871, 957, 987, 1019, 1163, 1243, 1261, 1501, 1539, 1633, 1843, 1957, 2037, 2213, 2273, 2339, 2441, 2511, 2639, 2741, 2753
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OFFSET
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1,1
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COMMENTS
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Only squarefree numbers.
Like A284643 but using number of non-divisors.
Both k and k-d(k) must be odd to be eligeable for this sequence. This means that d(k) is even. Therefore, this sequence and A284643 cannot share common terms.
For 177, 573, 597, 687, 4809, 6223, 7693, 24069, etc. are prime k-d(k) and k+d(k) too.
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LINKS
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EXAMPLE
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d(177) = 4 and concat(177,177-4) = 177173 and concat(177-4,177) = 173177 are both prime (like also 177 - 4 = 173 and 177 + 4 = 181).
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MAPLE
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select(n->isprime(n*10^(ilog10(n-tau(n))+1)+n-tau(n)) and isprime((n-tau(n))*10^(ilog10(n)+1)+n), [$3..2753]);
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PROG
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(PARI) isok(k) = my(sk = Str(k), sdk = Str(k-numdiv(k))); isprime(eval(concat(sk, sdk))) && isprime(eval(concat(sdk, sk))); \\ Michel Marcus, Apr 20 2018
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CROSSREFS
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KEYWORD
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nonn,base,easy
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AUTHOR
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STATUS
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approved
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