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for J$_{2}$ = 0 . 3, at T ≈ 1 . 25 (RGM data). The faster decay of the spin-spin correlators due to frustration is related to the decrease in the correlation length ξ with increasing J$_{2}$ , see the discussion below. 

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The uniform static spin susceptibility χ and the cor- relation length ξ depicted in Figs. 3 and 4 show a sim- ilar behavior. Due to the ferromagnetic ground state, both quantities diverge at T = 0 exponentially, see be- low. With increasing frustration J$_{2}$ the rapid increase in both quantities is shifted to lower temperatures. As shown in the insets of Figs. 3 and 4, at a certain fixed 

temperature both χ and ξ decrease rapidly with J$_{2}$ . 

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Since the susceptibility (together with the correlation length) is an important quantity to analyze the critical properties for T → 0, we first test the quality of our RGM approach in more detail by comparing the RGM data for N = 16 and N = 20 with ED data for the same system sizes, see Fig. 5. It is obvious, that the RGM and the ED data for χ are in excellent agreement. This is consistent with our previous finding 28 that for the 1D Heisenberg ferromagnet the RGM data for χ ( T → 0) and ξ ( T → 0) are in perfect agreement with exact Bethe ansatz results. Moreover, the RGM calculations of χ for finite N allow to estimate the magnitude of the finite-size effects. As can be clearly seen in Fig. 5, the finite-size effects at T ≲ 0 . 6 ( T ≲ 0 . 4) for J$_{2}$ = 0 . 1 ( J$_{2}$ = 0 . 3) be- come very large. Since the correlation length ξ becomes smaller with increasing J$_{2}$ (see the inset in Fig. 4), the finite-size effects become less pronounced with growing frustration. Nevertheless, based on our data we argue that the finite systems of N = 16 and N = 20, accessible 

