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A124886 3-almost prime triangle, read by rows. 2
1, 1, 7, 1, 11, 9, 1, 17, 3, 5, 1, 19, 8, 4, 14, 1, 26, 2, 6, 12, 15, 1, 27, 18, 10, 20, 30, 22, 1, 29, 13, 31, 21, 23, 40, 28, 1, 41, 25, 38, 32, 34, 16, 36, 39, 1, 43, 33, 35, 57, 42, 24, 44, 48, 50 (list; table; graph; refs; listen; history; text; internal format)
OFFSET

1,3

COMMENTS

This is to 3-almost primes (A014612) as A124883 is to semiprimes (A001358). The n-th row is of length n. Each value is the smallest previously unused natural number such that every pair of adjacent values in the triangle is 3-almost prime (A014612). Consider row 2. Starting with T(1,2) = 1, the least integer we can add to 1 and get a 3-almost prime is 7, since 1 + 8 = 8 = 2^3 is 3-almost prime. Consider row 3. Starting with T(1,3) = 1, the least integer we can add to 1 and get a 3-almost prime is 7, but we've already used that. The least unused integer that works is 11, since 1 + 11 = 12 = 2^2 * 3 is 3-almost prime. If we cross out ones from the triangle read by rows, what remains is a permutation of the natural number greater than 1. That is, every nonnegative integer appears in the triangle. The second column T(n,2) is monotone increasing.

REFERENCES

R. K. Guy, Unsolved Problems in Number Theory, 2nd ed. New York: Springer-Verlag, p. 106, 1994.

M. J. Kenney, "Student Math Notes." NCTM News Bulletin. Nov. 1986.

LINKS

Table of n, a(n) for n=1..55.

Eric Weisstein's World of Mathematics, Prime Triangle.

FORMULA

T(n,1) = 1 for all natural numbers n. For n>1 and 1<k<n we have T(n,k) = min{j such that j<>T(n,i) for i<k and j<>T(r,s) for r<n and for all i<j we have T(i,j) + T(i,j-1) is in A014612).

EXAMPLE

Triangle begins:

1.

1..7.

1.11..9.

1.17..3..5.

1.19..8..4.14.

1.26..2..6.12.15.

1.27.18.10.20.30.22.

1.29.13.31.21.23.40.28.

1.41.25.38.32.34.16.36.39.

1.43.33.35.57.42.24.44.48.50.

CROSSREFS

Cf. A001358, A014612, A036440, A051237, A124883.

Sequence in context: A124970 A251768 A178034 * A061195 A232111 A074283

Adjacent sequences:  A124883 A124884 A124885 * A124887 A124888 A124889

KEYWORD

easy,nonn,tabl

AUTHOR

Jonathan Vos Post, Nov 12 2006

STATUS

approved

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Last modified October 22 04:25 EDT 2019. Contains 328315 sequences. (Running on oeis4.)