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 A147616 Numbers k = concat(a,b) such that sigma(k) = sigma(a) * sigma(b), where sigma = A000203. 4
 38, 58, 66, 87, 118, 178, 205, 217, 275, 295, 298, 395, 451, 478, 492, 517, 538, 575, 660, 718, 766, 775, 838, 839, 870, 898, 1018, 1138, 1175, 1195, 1318, 1671, 1678, 1775, 1795, 1975, 2050, 2163, 2170, 2295, 2395, 2518, 2578, 2638, 2665, 2750, 2818 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS Concat(a,b) means decimal concatenation of a and b, i.e., a*10^[log_10(b)+1] + b, since we don't allow leading zeros in b. (However, allowing leading zeros in b would not give any additional term up to at least 10^6.) This sequence has been suggested by David Wilson on the SeqFan mailing list, Nov 08 2008. A possible variant would be to allow decomposition of k into an arbitrary number (>1) of substrings. If one requires decomposition of k into each of its digits, this yields A098771. LINKS Paolo P. Lava, Table of n, a(n) for n = 1..1000 EXAMPLE a(1)=38 is in the sequence since sigma(38) = 60 = 4*15 = sigma(3)*sigma(8). PROG (PARI) is_A147616(n)={ local(p=1, s=sigma(n)); while( n>p*=10, n%p*10

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Last modified September 19 13:45 EDT 2024. Contains 376012 sequences. (Running on oeis4.)