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%I #21 Nov 19 2020 18:36:14
%S 1,2,2,4,7,7,25,34,151,331,1690,3265,26449,64528,574513,1671208,
%T 16090657,54199564,559922497,2133503863,23506132363
%N a(n) = sum_of_digits(a(n-1)^a(n-2)) where a(1)=1 and a(2)=2.
%H GeeksforGeeks, <a href="https://www.geeksforgeeks.org/sum-digits-given-number-given-power">Sum of digits of a given number to a given power</a>
%F a(n) == 7 (mod 9) for n >= 5. - _Hugo Pfoertner_, Nov 15 2020
%e for n=6, a(6) = sum_of_digits(7^7) = sum_of_digits(823543) = 25
%t a[1]:=1; a[2]:=2; a[n_]:=Total[IntegerDigits[a[n-1]^a[n-2]]]; Array[a,19] (* _Stefano Spezia_, Nov 15 2020 *)
%o (SageMath)
%o a,b=1,2
%o L=[a,b]
%o for n in [1..17]:
%o c=b^a
%o c=sum(c.digits())
%o L.append(c)
%o a,b=b,c
%o print(L)
%o (PARI) a338917(nmax)={my(x=vector(nmax));x[1]=1;x[2]=2;for(k=3,nmax,x[k]=sumdigits(x[k-1]^x[k-2]));x};
%o a338917(18) \\ _Hugo Pfoertner_, Nov 15 2020
%Y Cf. A007953.
%K nonn,base,more
%O 1,2
%A _Sean Lestrange_, Nov 15 2020
%E a(20) from _Hugo Pfoertner_, Nov 15 2020
%E a(21) from _Chai Wah Wu_, Nov 19 2020
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