This site is supported by donations to The OEIS Foundation. Hints (Greetings from The On-Line Encyclopedia of Integer Sequences!)
 A147616 Numbers n = concat(a,b) such that sigma(n) = 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(b)+1] + b, since we don't allow leading zeros in b. (However, allowing leading zeros in b would not give any additional term at least up to 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 n into an arbitrary number (>1) of substrings. If one requires decomposition of n 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). MAPLE with(numtheory): P:=proc(q) local s, t, k, n; for n from 1 to q do for k from 1 to ilog10(n) do s:=n mod 10^k; t:=trunc(n/10^k); if s*t>0 then if sigma(s)*sigma(t)=sigma(n) then print(n); break; fi; fi; od; od; end: P(10^5); # Paolo P. Lava, Jan 27 2015 PROG (PARI) is_A147616(n)={ local(p=1, s=sigma(n)); while( n>p*=10, n%p*10

Lookup | Welcome | Wiki | Register | Music | Plot 2 | Demos | Index | Browse | More | WebCam
Contribute new seq. or comment | Format | Style Sheet | Transforms | Superseeker | Recent
The OEIS Community | Maintained by The OEIS Foundation Inc.

Last modified October 14 17:31 EDT 2019. Contains 328022 sequences. (Running on oeis4.)