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A122964
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Semiprimes with triangular indices.
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0
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4, 9, 15, 26, 39, 58, 85, 111, 134, 166, 205, 237, 291, 327, 382, 445, 497, 545, 623, 697, 766, 841, 921
(list; graph; refs; listen; history; internal format)
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OFFSET
| 1,1
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FORMULA
| a(n) = semiprime(T(1)), where T(n) is the n-th triangular number. a(n) = A001358(A000217(n)) = A001358(n*(n+1)/2).
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EXAMPLE
| a(1) = semiprime(T(1)) = semiprime(1) = 4 = 2^2.
a(2) = semiprime(T(2)) = semiprime(3) = 9 = 3^2.
a(3) = semiprime(T(3)) = semiprime(6) = 15 = 3 * 5.
a(4) = semiprime(T(4)) = semiprime(10) = 26 = 2 * 13.
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CROSSREFS
| Cf. A000217, A001358.
Sequence in context: A030664 A070160 A056928 * A178379 A070447 A106548
Adjacent sequences: A122961 A122962 A122963 * A122965 A122966 A122967
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KEYWORD
| easy,nonn
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AUTHOR
| Jonathan Vos Post (jvospost3(AT)gmail.com), Oct 27 2006
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