A063380 a(1) = 4, a(n) = concatenation of two closest factors of a(n-1) whose product equals a(n-1) or if a(n-1) is a prime then the concatenation of 1 and a(n-1).

4, 22, 211, 1211, 7173, 9797, 97101, 910789, 1182799, 1319029, 6719687, 7678761, 32559587, 257126691, 1591617149, 6653239233, 62767105999, 126149775659, 432933715713, 2435717774509, 6598336914323, 19495633384521

Offset

1,1

Links

Table of n, a(n) for n=1..22.

Mathematica

f[ n_Integer ] := (d = Divisors[ n ]; l = Length[ d ]; If[ EvenQ[ l ], ToExpression[ ToString[ d[ [ l/2 ] ] ] <> ToString[ d[ [ l/2 + 1 ] ] ] ], ToExpression[ ToString[ d[ [ l/2 + .5 ] ] ] <> ToString[ d[ [ l/2 + .5 ] ] ] ] ] ); NestList[ f, 4, 24 ]

Crossrefs

Cf. A003681, A062094 and A062095.

Sequence in context: A280828 A103437 A215201 * A113385 A356285 A120482

Adjacent sequences: A063377 A063378 A063379 * A063381 A063382 A063383

Keyword

base,nonn

Author

Robert G. Wilson v, Aug 08 2001

Status

approved