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A239713
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Primes of the form m = 3^i + 3^j - 1, where i > j >= 0.
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2
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3, 11, 29, 83, 89, 107, 251, 269, 809, 971, 2213, 2267, 6563, 6569, 6803, 8747, 19709, 19763, 20411, 59051, 65609, 177173, 183707, 531521, 538001, 590489, 1594331, 1594403, 1595051, 1596509, 4782971, 4782977, 4783697, 14348909, 14349149, 14526053, 14880347
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OFFSET
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1,1
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COMMENTS
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The base-3 representation of a term 3^i + 3^j - 1 has base-3 digital sum = 1 + 2*j == 1 (mod 2).
In base-3 representation the first terms are 10, 102, 1002, 10002, 10022, 10222, 100022, 100222, 1002222, 1022222, 10000222, 10002222, 100000002, 100000022, 100022222, 102222222, 1000000222, 1000002222, 1000222222, 10000000002, 10022222222, 100000000222, 100022222222, ...
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LINKS
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EXAMPLE
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a(1) = 3, since 3 = 3^1 + 3^0 - 1 is prime.
a(5) = 89, since 89 = 3^4 + 3^2 - 1 is prime.
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PROG
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(Smalltalk)
Answer: a(n)"
| a b i j k p q terms |
terms := OrderedCollection new.
k := 0.
b := 3.
p := b.
i := 1.
[k < self] whileTrue:
[j := 0.
q := 1.
[j < i and: [k < self]] whileTrue:
[a := p + q - 1.
a isPrime
ifTrue:
[k := k + 1.
terms add: a].
q := b * q.
j := j + 1].
i := i + 1.
p := b * p].
^terms at: self
--------------------
(Smalltalk)
"Version 2: Answers the n-th term of A239713.
Uses distinctPowersOf: b from A018900
Answer: a(n)”
| a k n terms |
terms := OrderedCollection new.
n := 1.
k := 0.
[k < self] whileTrue:
[(a:= (n distinctPowersOf: 3) - 1)
isPrime ifTrue: [k := k + 1.
terms add: a].
n := n + 1].
^terms at: self
-----------
(Smalltalk)
"Version 3: Answer an array of the first n terms of A239713.
Uses method primesWhichAreDistinctPowersOf: b withOffset: d from A239712.
Answer: #(3 11 29 ... ) [a(1) ... a(n)]”
^self primesWhichAreDistinctPowersOf: 3 withOffset: -1
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CROSSREFS
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KEYWORD
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nonn
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AUTHOR
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STATUS
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approved
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