What's Special About 256409?

 My diurnal age today, 26939, has the property that:

2 x 26939 + 3 = 53881 is prime

4 x 26939 + 5 = 107761 is prime

6 x 26939 + 7 = 161641 is prime

8 x 26939 + 9 = 215521 is prime

It thus belongs to a sequence of numbers \(n\) with the property that  \(2n+3\), \(4n+5 \), \(6n+7\) and \( 8n+9\) are all prime (A105653). The initial members of the sequence are:

164, 764, 1529, 2129, 2474, 3419, 5414, 7694, 9059, 11504, 12704, 13019, 15884, 16649, 20054, 20744, 22529, 24914, 26939, 29669, 32924, 35069, 36884, 39269

It's interesting to see how far we can extend this property. How many numbers will also yield a \(10n+11\) that is prime? Extending the range to one million, it can be seen that a quite a few numbers do qualify. They are:

5414, 12704, 13019, 44369, 82949, 98279, 105524, 112199, 115139, 123854, 134249, 134459, 187739, 188744, 210164, 225704, 247169, 256409, 296309, 302084, 367874, 375644, 382889, 399584, 404039, 476339, 487829, 526844, 532094, 566429, 578084, 766184, 779789, 787709, 854174, 883889, 919334, 966839

What about \(12n+13\) as well? The result is quite a few less. In fact only 12704, 13019, 105524, 256409 and 966839 qualify.

256409

When we try \(14n+15\), there is only one man left standing and that is 256409. Can this number go one further to \(16n+17\)? Indeed it can but at \(18n+19\), it fails. Here is a list of the primes produced along with the final composite number (all end in the digit 1) where "True" represents a prime number and "False" represents a composite number (permalink).

• 2 x 256409 + 3 = 512821 True
• 4 x 256409 + 5 = 1025641 True
• 6 x 256409 + 7 = 1538461 True
• 8 x 256409 + 9 = 2051281 True
• 10 x 256409 + 11 = 2564101 True
• 12 x 256409 + 13 = 3076921 True
• 14 x 256409 + 15 = 3589741 True
• 16 x 256409 + 17 = 4102561 True
• 18 x 256409 + 19 = 4615381 False

So it is for this reason that 256409 is rather special, at least in the range of positive integers up to one million. It is in fact the first member of OEIS A105657 containing numbers with the same property as 256409 but none of them can be the first! Here are the initial members of the sequence:

256409, 11120339, 13243229, 49798979, 296504669, 510578774, 520649219, 640598279, 674992499, 713074004, 830453714, 947378984

It can be noted that while the initial members all end in 9, the last three listed all end in 4. Even so the primes produced still end in 1 as 2 x 4 + 3 = 11 and 2 x 9 + 3 = 21 etc.. Take the final member listed, 947378984, as an example:

• 2 x 947378984 + 3 = 1894757971 True
• 4 x 947378984 + 5 = 3789515941 True
• 6 x 947378984 + 7 = 5684273911 True
• 8 x 947378984 + 9 = 7579031881 True
• 10 x 947378984 + 11 = 9473789851 True
• 12 x 947378984 + 13 = 11368547821 True
• 14 x 947378984 + 15 = 13263305791 True
• 16 x 947378984 + 17 = 15158063761 True
• 18 x 947378984 + 19 = 17052821731 False

There's no reason to suppose that there are not numbers out there that would extend the primes generated to \(18n+19\) and beyond. Using a Jupyter Notebook, a search to ten million produced nothing and, extending the search to one hundred million, the Notebook experienced a meltdown. So for the time being, 947378984 remains the largest member of the sequence and 256409 its smallest.

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Another interesting property of 256409 is that it has no repeating digits and, of the members of OEIS A105657 listed previously, it is the only such number. All the other numbers have at least one repeating digit. This is not all that surprising given that the other numbers have eight and nine digits and so the probability of a repeating digit is high. Any six digit number such as 256409, if digits are assigned randomly, will have a smaller probability of digits repeating. 

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256409 is a sphenic number which means that it has three distinct prime factors, in this case 43, 67 and 89. Now if the primes between 43 and 89 are listed, we see the following:

43, 47, 53, 59, 61, 67, 71, 73, 79, 83, 89

There are four primes between 43 and 67 and four primes also between 67 and 89. How often does this symmetry occur in sphenic numbers? We might ask it in the following way:

If \(n\) is a sphenic number with factors \(p_1\), \(p_2\) and \(p_3\), what numbers have the property that their primes indices are in arithmetic progression?

For example, the indices of 43, 67 and 89 are 14, 19 and 24 respectively and the latter three numbers are in arithmetic progression.

Well, in the range up to one million, there are 206964 sphenic numbers, a little over 20%. In that range only 601 satisfy the previously mentioned criteria and as we have seen 256409 is one of them. If we specify that the common difference must be 5, then only 21 numbers satisfy and these are (permalink):

806, 1887, 3895, 6923, 14993, 21359, 37111, 47519, 66263, 96773, 119939, 172457, 207583, 256409, 323689, 390769, 480083, 541741, 649967, 778231, 936371