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 A331590 Square array A(n,k) = A225546(A225546(n) * A225546(k)), n >= 1, k >= 1, read by descending antidiagonals. 14
 1, 2, 2, 3, 3, 3, 4, 6, 6, 4, 5, 8, 5, 8, 5, 6, 10, 12, 12, 10, 6, 7, 5, 15, 9, 15, 5, 7, 8, 14, 10, 20, 20, 10, 14, 8, 9, 12, 21, 24, 7, 24, 21, 12, 9, 10, 18, 24, 28, 30, 30, 28, 24, 18, 10, 11, 15, 27, 18, 35, 15, 35, 18, 27, 15, 11, 12, 22, 30, 36, 40, 42, 42, 40, 36, 30, 22, 12, 13, 24, 33, 40, 45, 20, 11, 20, 45, 40, 33, 24, 13 (list; table; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS As a binary operation, this sequence defines a commutative monoid over the positive integers that is isomorphic to multiplication. The self-inverse permutation A225546(.) provides an isomorphism. This monoid therefore has unique factorization. Its primes are the even terms of A050376: 2, 4, 16, 256, ..., which in standard integer multiplication are the powers of 2 with powers of 2 as exponents. In this monoid, in contrast, the powers of 2 run through the squarefree numbers, the k-th power of 2 being A019565(k). 4 is irreducible and its powers are the squares of the squarefree numbers, the k-th power of 4 being A019565(k)^2 (where "^2" denotes standard integer squaring); and so on with powers of 16, 256, ... In many cases the product of two numbers is the same here as in standard integer multiplication. See the formula section for details. LINKS Antti Karttunen, Antidiagonals n = 1..144, flattened Eric Weisstein's World of Mathematics, Monoid FORMULA Alternative definition: A(n,1) = n; A(n,k) = A(A059897(n,k), A003961(A059895(n,k))). Main derived identities: (Start) A(n,k) = A(k,n). A(1,n) = n. A(n, A(m,k)) = A(A(n,m), k). A(m,m) = A003961(m). A(n^2, k^2) = A(n,k)^2. A(A003961(n), A003961(k)) = A003961(A(n,k)). A(A019565(n), A019565(k)) = A019565(n+k). (End) Characterization of conditions for A(n,k) = n * k: (Start) The following 4 conditions are equivalent: (1) A(n,k) = n * k; (2) A(n,k) = A059897(n,k); (3) A(n,k) = A059896(n,k); (4) A059895(n,k) = 1. If gcd(n,k) = 1, A(n,k) = n * k. If gcd(n,k) = 1, A(A225546(n), A225546(k)) = A225546(n) * A225546(k). The previous formula implies A(n,k) = n * k in the following cases: (1) for n = A005117(m), k = j^2; (2) more generally for n = A005117(m_1)^(2^i_1), k = A005117(m_2)^(2^i_2), with A004198(i_1, i_2) = 0. (End) EXAMPLE From Antti Karttunen, Feb 02 2020: (Start) The top left 16 X 16 corner of the array:    1,  2,  3,  4,  5,  6,   7,   8,   9,  10,  11,  12,  13,  14,  15,  16, ...    2,  3,  6,  8, 10,  5,  14,  12,  18,  15,  22,  24,  26,  21,  30,  32, ...    3,  6,  5, 12, 15, 10,  21,  24,  27,  30,  33,  20,  39,  42,   7,  48, ...    4,  8, 12,  9, 20, 24,  28,  18,  36,  40,  44,  27,  52,  56,  60,  64, ...    5, 10, 15, 20,  7, 30,  35,  40,  45,  14,  55,  60,  65,  70,  21,  80, ...    6,  5, 10, 24, 30, 15,  42,  20,  54,   7,  66,  40,  78,  35,  14,  96, ...    7, 14, 21, 28, 35, 42,  11,  56,  63,  70,  77,  84,  91,  22, 105, 112, ...    8, 12, 24, 18, 40, 20,  56,  27,  72,  60,  88,  54, 104,  84, 120, 128, ...    9, 18, 27, 36, 45, 54,  63,  72,  25,  90,  99, 108, 117, 126, 135, 144, ...   10, 15, 30, 40, 14,  7,  70,  60,  90,  21, 110, 120, 130, 105,  42, 160, ...   11, 22, 33, 44, 55, 66,  77,  88,  99, 110,  13, 132, 143, 154, 165, 176, ...   12, 24, 20, 27, 60, 40,  84,  54, 108, 120, 132,  45, 156, 168,  28, 192, ...   13, 26, 39, 52, 65, 78,  91, 104, 117, 130, 143, 156,  17, 182, 195, 208, ...   14, 21, 42, 56, 70, 35,  22,  84, 126, 105, 154, 168, 182,  33, 210, 224, ...   15, 30,  7, 60, 21, 14, 105, 120, 135,  42, 165,  28, 195, 210,  35, 240, ...   16, 32, 48, 64, 80, 96, 112, 128, 144, 160, 176, 192, 208, 224, 240,  81, ... (End) PROG (PARI) up_to = 1275; A019565(n) = factorback(vecextract(primes(logint(n+!n, 2)+1), n)); A048675(n) = { my(f = factor(n)); sum(k=1, #f~, f[k, 2]*2^primepi(f[k, 1]))/2; }; A331590sq(x, y) = if(1==x, y, if(1==y, x, my(fx=factor(x), fy=factor(y), u=max(#binary(vecmax(fx[, 2])), #binary(vecmax(fy[, 2]))), prodsx=vector(u, x, 1), m=1); for(i=1, u, for(k=1, #fx~, if(bitand(fx[k, 2], m), prodsx[i] *= fx[k, 1])); for(k=1, #fy~, if(bitand(fy[k, 2], m), prodsx[i] *= fy[k, 1])); m<<=1); prod(i=1, u, A019565(A048675(prodsx[i]))^(1<<(i-1))))); A331590list(up_to) = { my(v = vector(up_to), i=0); for(a=1, oo, for(col=1, a, i++; if(i > up_to, return(v)); v[i] = A331590sq(col, (a-(col-1))))); (v); }; v331590 = A331590list(up_to); A331590(n) = v331590[n]; \\ Antti Karttunen, Feb 02 2020 CROSSREFS Isomorphic to A003991 with A225546 as isomorphism. Cf. A003961(main diagonal), A048675, A059895, A059896, A059897. Rows/columns, sorted in ascending order: 2: A000037, 3: A028983, 4: A252849. A019565 lists powers of 2 in order of increasing exponent. Powers of k, sorted in ascending order: k=2: A005117, k=3: A056911, k=4: A062503, k=5: A276378, k=6: intersection of A325698 and A005117, k=7: intersection of A007775 and A005117, k=8: A062838. Irreducibles are A001146 (even terms of A050376). Sequence in context: A173910 A036846 A227396 * A326496 A058740 A160642 Adjacent sequences:  A331587 A331588 A331589 * A331591 A331592 A331593 KEYWORD nonn,tabl AUTHOR Peter Munn, Jan 21 2020 STATUS approved

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Last modified August 14 10:47 EDT 2020. Contains 336480 sequences. (Running on oeis4.)